exploring the universe with the low frequency array … · page1of59 exploring the universe with...

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EXPLORING THE UNIVERSE WITHTHE LOW FREQUENCY ARRAY

A SCIENTIFIC CASEVersion 1.00 – 25-September-2002

1 Introduction ...............................................................................................................................31.1 Background - Exploring the Low-Frequency Universe....................................................31.2 Scientific Objectives of LOFAR ........................................................................................51.3 Low-Frequency Radio Radiation: Unique Physical Diagnostics.....................................51.4 A Census of the Low-Frequency Sky...............................................................................71.5 Seeking Variable Sources ................................................................................................9

2 REIONIZATION OF THE UNIVERSE ...................................................................................102.1 Introduction and key questions.......................................................................................102.2 Constraints on the Epoch of Reionization......................................................................112.3 Constraints on the Sources of Reionization. .................................................................122.4 Optimizing LOFAR for Studying Reionization................................................................13

2.4.1 Intensity....................................................................................................................132.4.2 Expected Angular Scales........................................................................................132.4.3 Implications for LOFAR...........................................................................................14

2.5 Answers from LOFAR to Key Questions .......................................................................153 FORMATION AND EVOLUTION OF GALAXIES, CLUSTERS AND ACTIVE NUCLEI .....17

3.1 Introduction and Key Questions .....................................................................................173.2 Distant Radio Galaxies: Probes of Massive Galaxy and Cluster Formation................173.3 Distant starburst galaxies: Probes of galaxy evolution. ................................................203.4 Diffuse Radio Sources: Probes of Intergalactic Gas Evolution.....................................23

3.4.1 Cluster Radio Halos: Tracing Over-dense Regions...............................................243.4.2 Giant radio sources: Tracing under-dense regions. ..............................................25

3.5 Gamma Ray Bursters – Prompt Emission and Afterglows ...........................................263.6 Answers from LOFAR to key questions.........................................................................26

4 THE HIGHEST ENERGY PHENOMENA..............................................................................294.1 LOFAR as a Cosmic Ray Detector ................................................................................29

4.1.1 Background Information: Cosmic Rays..................................................................294.2 Gravitational Waves - LIGO events................................................................................33

4.2.1 An Unbiased Survey for Transient Astronomical Radio Sources (STARE)..........334.2.2 The Physics of Collapse and Explosion .................................................................33

4.3 LOFAR as an All-Sky Monitor ........................................................................................354.3.1 Particle acceleration in cosmic explosions.............................................................364.3.2 New emission regimes at LOFAR frequencies ......................................................39

5 THE MILKY WAY AND NEIGHBOURING GALAXIES.........................................................425.1 Testing the Standard Shock Acceleration Model ..........................................................425.2 Thermal and Nonthermal Emission in Nearby Galaxies ...............................................435.3 Supernova Remnants and the Distribution of Ionized Gas in the ISM.........................445.4 Discovering New SNRs ..................................................................................................44

5.4.1 Spectral Studies ......................................................................................................445.4.2 Absorption & ISM Studies .......................................................................................445.4.3 Extragalactic SNR Studies......................................................................................45

5.5 H II Regions.....................................................................................................................465.6 Interstellar Propagation Effects ......................................................................................465.7 Polarimetry ......................................................................................................................485.8 Recombination Lines ......................................................................................................495.9 Neutron Stars and Pulsars .............................................................................................49

5.9.1 Searching for new pulsars ......................................................................................49

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5.9.2 Emission physics.....................................................................................................505.10 Jupiter and Extrasolar Jupiters ...................................................................................52

5.10.1 Jupiter Magnetosphere........................................................................................525.10.2 Decameter Bursts................................................................................................535.10.3 Extrasolar Planets ...............................................................................................53

6 THE SUN AND SOLAR-TERRESTIAL RELATIONSHIPS...................................................556.1 Introduction......................................................................................................................556.2 Space Weather ...............................................................................................................56

6.2.1 LOFAR and Coronal Mass Ejections......................................................................566.2.2 The Solar Magnetic Cycle and the Terrestrial Climate..........................................566.2.3 LOFAR and CME warning ......................................................................................576.2.4 LOFAR, Active Radar and Space Weather............................................................58

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1 Introduction

1.1 Background - Exploring the Low-Frequency UniverseDuring the last half century our knowledge of the Universe has been revolutionized by theopening of observable windows outside the narrow visible region of the spectrum. Radiowaves, infrared and ultraviolet radiation and X- and gamma rays have provided new andcompletely unexpected information about the nature and history of the Universe and haveresulted in the discovery of a cosmic zoo of strange and exotic objects. One of the fewspectral windows that still remain to be explored is at the low radio frequencies, the lowestenergy extreme of the accessible spectrum. LOFAR, the Low Frequency Radio Array, is alarge radio telescope that will open this virgin territory to a broad range of astrophysicalstudies.

The mission of LOFAR is to survey the Universe at frequencies of from ~ 10 – 240 MHz(corresponding to wavelengths of 1.5 – 30 m). Radio astronomy was born at thesewavelengths in 1931, when Karl Jansky investigated the background noise that was plaguingtransatlantic short-wave communications. Since then, low frequency radio astronomy hasbeen neglected because of the poor resolving power of the available facilities and thedisturbing effects of the ionosphere on observations.

Because the spatial resolution of a telescope is proportional to its operating frequency, radiotelescopes such as the Westerbork Radio Synthesis Telescope have extremely poor spatialresolution when operated at low frequencies. The radio images obtained by low frequencyradio facilities (resolutions of arcminutes) are blurred by a factor of several thousandcompared with optical pictures of the sky. The blurred images result in a so-called "confusion"effects that have limited the sensitivity at low radio frequencies to the level of ~1 Jansky1 andthe number of objects that can be studied to the brightest few hundred.

The resolution of a radio telescope can be improved by enlarging the aperture, or in the caseof a Westerbork-type array of antennas, by increasing the maximum distance between theelements of the array - i.e. the “baseline”. At low frequencies, to achieve useful resolutionscomparable with visible images of the sky, maximum baselines of several hundred kilometresare needed.Until now, one of the most important limitations in achieving such long baselines at low radiofrequencies has been the complicated structure of the ionosphere and its variation over time.Just as the atmosphere causes stars to twinkle, the irregularities in the ionosphere producejittering in the radio images.

There have been several recent technological developments that make it possible tocompensate for the jittering effects of the ionosphere. First, the large increase of computerpower makes it possible to create and process images of very wide fields on short enoughtimescales to monitor and correct for the ionospheric jitter. Second, progress in antennadesign have made it possible to construct low-frequency antennas with several simultaneousbeams that can be pointed to and used to monitor different regions of the sky simultaneously.Combining these developments with interferometer calibration algorithms that have beendeveloped during the past 3 decades in the Netherlands and the USA will allow theconstruction of a radio array with baselines extending to a few hundred kilometres, operatingat frequencies down to ~10 MHz. Such an array will for the first time probe the distantUniverse at the low-energy extremity of the electromagnetic spectrum.LOFAR will consist of an array of antennas spread over a ~ 400 km region, that will providesufficient resolution to allow radio sources to be identified with visible objects, even at lowfrequencies. Occupying its central 2 km will be a more densely filled "virtual" core (VC), thatwill allow more effective calibration of the instrument and optimize its sensitivity for a specialexperiment to study the reionization phase of the Universe. The design will provide fast (~1ms) frequency selection and pointing, capable of rapidly imaging radio sources across the skyand spectrum. Multiple independent beams will herald a new technological approach to

1 1 Jansky (Jy) = 10-26 W m-2 Hz-1

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observing, yielding unprecedented flexibility when compared with higher cost, higherfrequency ground or space-based systems.

The power of LOFAR is specified in Table 1.1 and illustrated in Figure 1.1, where sensitivityand spatial resolution are plotted against frequency. The limits for previous low-frequencyfacilities are compared with that of LOFAR.

Figure 1.1 Comparison of LOFAR capabilities with those of previous facilities as a function offrequency. The sensitivity after one hour of integration (left) and angular resolution (right) of LOFAR areshown in red together with those of previous low frequency radio survey facilities.

Frequency Range Low 10-90 MHzHigh 110-240 MHz

Effective Collecting Area Low (10-90 MHz) 106.1 (freq./15 MHz)-2 m2

High (110-200 MHz) 105.1 m2

High (200-240 MHz) 105.1 (freq./200 MHz)-2 m2

Configuration Virtual Core (VC) (25%) < ~ 2 kmExtended Array 400 km

Sensitivity Point Source Brightness TemperatureVC Full Array VC Full Array

10 MHz 12 mJy 3.0 mJy 20 K 105.3 K30 MHz 6.5 mJy 1.6 mJy 12 K 105.1 K75 MHz 4.0 mJy 1.0 mJy 7.3 K 10 4.6 K

120 MHz 0.5 mJy 0.13 mJy 0.9 K 10 4.0 K200 MHz 0.14 mJy 0.03 mJy 0.25 K 10 3.4 K

Greatest InstantaneousSky Coverage

0.1 sterad/beam at 10 MHz

4 sterad (virtual core)Highest AngularResolution (FWHM)

0.64 arcsec at 240 MHz

Number of SpectralChannels

4096

Time Resolution 1 msPolarization full Stokes

# Sensitivity calculations are for a single polarization, 1 hour integration, 4 MHz bandwidthTable 1.1 Specifications for LOFAR

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1.2 Scientific Objectives of LOFARThe sensitivities and spatial resolutions attainable with LOFAR will make possible severalfundamental new studies of the Universe as well as facilitating unique practical investigationsof the environment of the earth.

• In the very distant Universe (7 < z < 10), LOFAR can search for the signatureproduced by the reionization of neutral hydrogen. This crucial phase change ispredicted to occur at the epoch the formation of the first stars and galaxies, markingthe end of the so-called “dark ages”. The redshift at which reionization is believed tooccur will shift the 1420 MHz line of neutral hydrogen into the LOFAR observingwindow.

• In the distant “formative” Universe (1.5 < z < 7), LOFAR will detect the mostdistant massive galaxies and will study the processes by which the earliest structuresin the Universe (galaxies, clusters and active nuclei) form and probe the intergalacticgas.

• In the nearby Universe, LOFAR will map the 3-dimensional distribution of cosmicrays and global magnetic field in our own and nearby galaxies.

• The High Energy Universe, LOFAR will detect the ultra high energy cosmic rays asthey pierce the Earth’s atmosphere.

• Within our own galaxy, LOFAR will detect flashes of low-frequency radiation frompulsars and short-lived transient events produced by stellar merging and interactionsand will search for Jupiter-like extra-solar planets.

• Within our solar system, LOFAR will detect coronal mass ejections from the Sunand provide continuous large-scale maps of the solar wind. This crucial informationabout solar weather and its effect on the Earth will facilitate predictions of costly anddamaging geomagnetic storms.

• Within the Earth’s immediate environment, LOFAR will map irregularities in theionosphere continuously, detect the ionizing effects of distant gamma-ray bursts andthe flashes predicted to arise from the highest energy cosmic rays, whose origin of isunclear.

• By exploring a new spectral window LOFAR is likely to make unexpected"serendipitous" discoveries. Detection of new classes of objects and/or newastrophysical phenomena have resulted from almost all previous facilities that opennew regions of the spectrum, or pushed instrumental parameters, such as sensitivityby more than an order of magnitude (e.g. Harwit, M. Cosmic Discovery. HarvesterPress, 1981)

The key scientific drivers will be described in detail below. Additional LOFAR applications willinclude the study of supernova remnants and their interaction with the interstellar medium,interstellar propagation effects, interstellar radio recombination lines. Much LOFAR sciencebuilds on fundamental areas of research that have been pursued intensively or pioneeredwithin the Netherlands during the last half century.

1.3 Low-Frequency Radio Radiation: Unique PhysicalDiagnostics

Before discussing the scientific drivers of LOFAR in detail, we draw attention to the specialaspects of the radiation at the extremely low frequencies at which LOFAR will observe. Suchprocesses will provide diagnostics that LOFAR will exploit in pursuing its scientific goals. Astronomy in the visible is dominated by thermal radiation emitted by hot gas in stars andgalaxies. However, radio astronomy operates in a regime in which the dominant radiation isdue to other mechanisms. Several unique astrophysical diagnostics can be derived from

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radiation processes that occur at frequencies accessible to LOFAR. The following areamongst the relevant radiation mechanisms.

Synchrotron Emission. Synchrotron radiation is produced by electrons moving close to thespeed of light in a magnetic field. This is the dominant radiation mechanism encountered inclassical radio astronomy. Because the objects that emit luminous radio radiation (e.g. distantactive galaxies, e.g. Fig. 2) are different from those than luminous thermal radiation (e.g.bright stars and nebulae), the radio Universe appears very different from the visible Universe.During the last half century opening the radio Universe to astronomy therefore resulted inmany unexpected discoveries. Synchrotron sources that will be observed by LOFAR includelobes and jets emitted by the nuclei of most distant galaxies, and the cosmic rays andsupernova remnants produced by stars in normal galaxies.

Figure 1.2 The radio galaxy Cygnus A, one of the brightest objects in the radio sky appears radicallydifferent when observed at radio or optical wavelengths. Cygnus A is located at a distance of 600million light years. The lower frame shows a picture of the radio emission. On the upper frame this radioimage (red) is superimposed on a (blue) optical picture of the same part of the sky. Most of the opticalobjects are foreground stars. The elliptical galaxy responsible for producing the two enormous radiolobes is the fuzzy object close to the centre of the picture. The radio source is due to relativistic plasmaproduced by a rotating massive black hole deep at the centre of the galaxy. The radio size of ~140 kpc(500,000 light-years) is an order of magnitude larger than that of the central galaxy. LOFAR will makeobservations of the sky at wavelengths that are longer by a factor of ~100 than those at which this radiopicture was taken.

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Relatively little is known about the spectra of most synchrotron sources at frequencies lowerthan ~30 MHz. Because synchrotron-emitting electrons lose energy as they grow older,radiation at the lowest frequencies is produced by the oldest relativistic electrons (age ~108

y). LOFAR will therefore be able to study the death pangs of "fossil" radio sources andprovide new constraints on radio source evolution.

Coherent Plasma Emission Although the proposed new facility will observe large numbersof synchrotron sources, one of the most exciting aspects of LOFAR is that it will operate in alow-energy region of the spectrum that is sensitive to other little-studied radiation processes,such as coherent plasma emission and cyclotron emission. Coherent plasma emission isknown to be important in the Sun and Jupiter, the two brightest objects in the low-frequencyradio sky. Astronomical surveys made in regions unexplored spectral regions dominated byunstudied emission mechanisms have always revealed unexpected populations of objects.This is also likely to be the case with LOFAR.

Absorption Processes Many synchrotron sources have spectra that decline sharply at lowradio frequencies. This decline at low frequencies is usually attributed to absorption of thesynchrotron radiation either within the emitting object itself or in the path between the emitterand the earth. A study of this absorption can provide diagnostics about the densities andgeometry of gas and plasma inside the radio sources, the surrounding environment and thepath between the emitting source and the earth.

There are three main relevant processes.• Synchrotron self absorption, an internal effect whose cut-off frequency constrains the

sizes, geometries and magnetic field strengths of compact extragalactic and galacticradio sources.

• Thermal absorption, whose cut-off frequency constrains the density, temperature andgeometry of the interstellar medium in galaxies.

• The Razin-Tsytovich Effect, that operates when gas densities are sufficiently highand frequencies are sufficiently low that the index of refraction is less than unity. Therelevant cut-off frequency constrains the density, magnetic field strength andgeometry of the interstellar medium.

Extending observations of such phenomena to the lowest frequencies will improve thesensitivity of such diagnostic studies by up to an order of magnitude.

1.4 A Census of the Low-Frequency SkyOne of the most important techniques that will be used to pursue LOFAR's scientificobjectives will be to conduct unbiased surveys of large regions of the accessible sky atseveral frequencies. Large-sky surveys are the bedrock on which much of modernastrophysical knowledge has been built and have traditionally been major drivers of Dutchastronomy. For example, surveys play an important strategic role by providing catalogues ofunique objects that act as leverage in obtaining time at highly oversubscribed ground-basedand space facilities. Surveys such as the optical Palomar Observatory Sky Survey (POSS)and the radio 3C, Parkes and Westerbork surveys have provided material for at least 2 or 3decades of high-impact astrophysical discoveries.

In the early eighties the Netherlands played a major role in producing the highly successfulIRAS all-sky infrared survey that opened up a new spectral window, just as LOFAR will do.The IRAS surveys have had a fundamental impact over a broad range of galactic (e.g. Fig. 3)and extragalactic astrophysics and the early success of IRAS stimulated ESA to fund thefollow-up ISO mission. Early access to the IRAS survey played a major role in shaping Dutchastronomy during the subsequent 2 decades. LOFAR will be the IRAS of low-frequency radioastronomy.

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Figure 1.3 Infrared picture of the Milky Way as seen by the highly successful Dutch-US-UK surveysatellite IRAS that opened up the far IR region of the spectrum to a broad range of astrophysical studies(Beichman et al 1988). LOFAR will be a similar survey facility for exploration of the lowest energyaccessible region of the electromagnetic spectrum. Both IRAS and LOFAR use the extreme demands offundamental research to provide test-beds for frontier technological developments.

Although they did not open new spectral windows, recent productive radio surveys include theWesterbork surveys at 325 MHz (WENSS and WISH) and the NRAO VLA Sky Surveys at1400 MHz (NVSS and FIRST). Between them, these three surveys have resulted in thedetection of millions of radio sources. These surveys so far contributed to seven Dutch Ph.D.theses on topics such as studies of the large-scale structure of the Universe, the nature ofextra-galactic radio sources and their relation to galaxy formation and the use of galacticforeground polarization as a probe of the interstellar medium in our own galaxy.

LOFAR will exploit the experience gained in conducting such surveys to open up the lowfrequency radio window. By realizing a combined jump in sensitivity and resolution (see Fig.1), LOFAR will take radio astronomy into hitherto unexplored regions of parameter space.Just as IRAS sources were the basis of many successful follow-up proposals on large opticaltelescope, we expect that the properties of LOFAR sources will be studied by the highlyoversubscribed present and planned optical and infrared facilities, such as the VLT, Keck,HST, ALMA, SIRTF and the NGST.

LOFAR will survey and monitor the galactic and extragalactic sky at several frequencies. Withthe unique capability to observe with a number of synthesized beams simultaneously, it willbe possible to observe large areas of sky to unprecedented depths. Since the location ofparameter space that LOFAR will observe is unknown territory, an estimate of the number ofsources that LOFAR will observe is necessarily based on surveys of small regions of sky thathave been carried out at higher frequencies. Using such radio surveys as carried out with theWesterbork radio telescope at 1.4 GHz (Katgert et at 1988 and Garrett et al 2000), thenumber of sources per unit surface area on the sky can be calculated (see Table 2). If thesky is observed at 5 different illustrative LOFAR frequencies (10, 30, 75, 120 and 200 MHz)using 5 different beams, then after one year of continuous observing, significantly more thanone million radio sources will be detected at each frequency. We note that LOFAR surveyswill have a different character at low and high frequencies. At low frequencies, large parts ofthe sky can be surveyed, yielding important samples of very distant radio galaxies and clusterhalos. Repeated observations of these areas will allow for a systematic search for importantvariable objects including Jupiters orbiting nearby stars and afterglows of the mysteriousgamma-ray bursts. At high frequencies, the depth will be such that a large fraction of the radiosources will identified with distant starbursting galaxies. This will yield well defined samplesfor studying the evolution and spatial distribution of this important class of galaxy.

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One of the unique features of LOFAR will be its ability to survey the sky simultaneously withseveral thousand different frequency channels and a spectral resolution of < 1 KHz. This willallow the spectral signal of the reionization of the Universe to be studied and facilitateserendipitous detection of spectral lines in the LOFAR band. The LOFAR large-sky surveysand dedicated observations directed at special regions of the sky will form the basis forstudies in several key areas of science that will be described below in this document.

ObservingFrequency(MHz)

AngularResolution(arcsec)

Surveylimit 1

(10 _)(mJy)

Surface densityof sources(No. per arcmin)

Area coveredafter 1 year(sq. deg)

Total numberof sourcesafter 1 year

10 15 30 0.5 3000 5.4 million30 5.2 2 4 3000 4.3 million75 2.1 0.3 25 60 5.3 million120 1.3 0.1 66 62 15 million200 0.8 10 125 7.5 3.3 million

Table 1.2 Surveying the Sky with LOFAR

1 The survey limit is determined either by confusion or the feasible integration time. It isassumed that 5 independent LOFAR beams will be available.

1.5 Seeking Variable SourcesMany celestial objects are variable. Besides charting the static low-frequency sky, one of themain goals of LOFAR will be to search for and study variable objects. LOFAR variablesources are laboratories for studying some of the highest energy phenomena in the Universe.The ability to make large-sky surveys with LOFAR is therefore an important driver for theLOFAR design.

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2 REIONIZATION OF THE UNIVERSE

2.1 Introduction and key questionsAccording to present views of the early Universe (e.g see Figure 2.1), hydrogen recombinedinto its neutral state about half a million years after the Big Bang, when the primordial matterhad cooled to a temperature of about 3000 K. The Universe then entered a period of“darkness”, when its temperature became steadily lower due to the universal expansion.These “Dark Ages” (Rees 1996) ended many hundreds of million years later, when the firststars were formed and assembled into proto-galaxies. Ionizing radiation from these stars andembyo galaxies then began to warm the Universe and produce observable radiation. After asufficient number of ionizing sources had formed, the temperature and the ionized fraction ofthe Universe increased rapidly and most of the neutral hydrogen eventually disappeared. Thisperiod, in which the Universe went from a phase in which almost all the hydrogen was neutralto a state in which it was almost completely ionized, is referred to as the “reionization” of theUniverse. The moment that marked the time when 50% of the hydrogen in the Universe wasionized is termed the “Epoch of Reionization" (E-o-R) (e.g. see Gnedin, 2001). The seeds ofseveral properties of our present Universe, such as the observed web-like distribution ofgalaxies (Figure 2.2) are likely to have been embedded during this crucial period in history.

Figure 2.1 Cartoon of the likely development of the early Universe. About 500,000 years after the BigBang (z ~ 1000) hydrogen recombined and remained neutral for a few hundred million years (the “darkages”). At a redshift, z ~ 10, the first stars galaxies and quasars began to form, heating and reionizingthe hydrogen gas. LOFAR will be able to observe the global spectral signature of neutral hydrogen closeto this epoch of reionization (E-o-R).

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Figure 2.2 The cosmic web. Matter in the local Universe (hot and warm baryons) is known to lies in alarge-scale filamentary structuresand this cosmic web is believed to extend to the highest redshifts. Thispicture was taken from Cen & Ostriker 1999 and illustrates the results of numerical N-bodyhydrodynamical models. The seeds of the cosmic web were implanted during the dark ages, just priorto reionization. This primeval era will be accessible to LOFAR.

Three of the most important questions need to be addressed about this fundamental changein the state of the early Universe are:

1. When did the reionization of the Universe occur?

2. How rapidly did the reionization of the Universe progress?

3. What were the dominant sources of ionization and how did they affect the globalprogression of the reionization?

Neutral hydrogen comprises ~ 5% - 10% of the 'critical' density of the Universe (reference ?check) and is potentially one of the most important diagnostics of the final stages of the darkages of and the beginning stages of reionization (e.g. Rees, 1999).

2.2 Constraints on the Epoch of ReionizationComputer simulations imply that the process of reionization was rapid and took place at someepoch between redshifts z ~ 15 and z ~ 6 (Gnedin 2000, Tozzi et al. 2000). There areseveral observational constraints are consistent with these predictions and allow the E-o-R tobe further refined.

The spectra of the most distant quasars allow an upper limit to be placed on the epoch atwhich reionization occurred. It has long been known that the reionization of the Universe musthave occurred at epochs corresponding to a redshift of z > 5, because of the absence ofabsorption blueward of Ly α in the spectra of quasars out to that redshift. Such a cut-off in thecontinuum spectra would be produced by neutral hydrogen in the Universe prior toreionization (Gunn – Peterson effect – reference). During the last two years several quasarsand a few galaxies having 6.5 > z > 5 have been discovered with the Sloan Digital SkySurvey (Fan et al, 2001, Becker et al, 2001 Pentericci et al, Hu et al.2002). The spectra ofthese objects showed the long predicted “Gunn-Peterson” absorption. The implication of thisexciting result is that in the period corresponding to 6.5 > z > 5, the fraction of neutral

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hydrogen in the Universe decreased dramatically from > 10-4 to ~10-5. The epoch ofreionization is likely to have occurred in the period just before these quasars were observed.

The earliest time that reionization can have occurred can also be constrained observationally.First, the epoch of reionization must have taken place at redshifts, z < 30. Otherwise therewould be a suppression of the first Doppler peak in the angular fluctuation spectrum of theCosmic Microwave Background (Tegmark & Zaldarriaga 2000; De Bernardis et al. 2000). Thisis not observed. Secondly, measurements of the temperature of the intergalactic medium(Lyman α forest clouds) at redshift z ~ 3, imply that the reionization must have occurred afterz ~ 10. Because of the (adiabatic) expansion of the Universe, an earlier epoch of reionizationwould have resulted in a temperature of the z ~ 3 intergalactic medium that would be coolerthan observed (Theuns et al, 2002; Hui et al, 2001).

In summary, the observational constraints and numerical simulations all imply that most of theneutral hydrogen in the Universe must have become ionized during the epoch correspondingto a redshift range of between z ~ 10 and z ~ 6.

If the 21 cm line emitted by neutral hydrogen (van der Hulst 1948??) just before reionizationwill be redshifted by the expansion of the Universe from its rest frequency of 1420 MHz towithin the frequency range between 100 and 200 MHz (corresponding to redshifts of 6-13).Observations at LOFAR radio frequencies can therefore offer a direct view of the primevalhydrogen gas, which eventually formed into 'visible' stars, galaxies and clusters. LOFAR willbe the first telescope to operate in the frequency range of 100-200 MHz that has enoughsensitivity and dynamic range to detect and study this neutral hydrogen signal. The ability tostudy the signal of reionization has therefore played an important role in driving the arrayconfiguration design of LOFAR as well as the frequency range to be covered by its receivers.

2.3 Constraints on the Sources of Reionization.At least three different types of sources have been proposed as contributing to the radiationthat reionized the Universe: (i) emission from the first generation of stars, (ii) radiationreleased in the collapse of the first gas clouds that formed sub-galactic fragments, and (iii)emission from an early generation of active galactic nuclei and quasars.

The powerful ionizing ultra-violet and X-ray continua of quasars were long thought to play thedominant role in the ionization of the Universe, because they appear to be responsible for thebulk of the ionizing background emission at low redshifts (z < 4). However, the space densityof bright quasars at high redshifts is now known to be insufficient by a large factor to make asignificant contribution to the global reionizing flux (e.g. Madau, 2000). Nevertheless, theymay have played a role in “preheating” the intergalactic medium.

Present evidence favours emission from stars as the dominant factor in reionization. In recentyears systematic deep multicolour surveys for have uncovered a population of objects atredshifts of up to ~ 6.5 (e.g. Steidel et al 200X, Malhotra and Rhoads, 2001). These objectsare termed “Lyman-break” galaxies, because of the characteristic cutoff in their ultravioletspectra shortward of Ly-α, due to absorption by intrinsic neutral hydrogen and are known tobe undergoing star formation.

The first stars were probably more massive (> 100 Msun, e.g. Abel et al. 2002) and hotter thanthe bulk of the stars forming in present-day galaxies. These young stars would producecopious amounts of ultraviolet photons, that would ionize the surrounding hydrogen, in so-called “Stromgren spheres”. When the Stromgren spheres from collections of young stars inthe first mini-galaxies start to overlap, a dense patchwork of ionized regions, separated byneutral gas, will develop (Gnedin and Ostriker, 1997; Gnedin, 2000). This ionized mosaic willgradually spread until the whole Universe becomes ionized.

A complication in this scenario for reionization is the possible role played by gas cloudsaround the first galaxies (“mini-halos”) in shielding the Universe from exposure to the firstgeneration of ionizing stars (Barkana and Loeb, 2000; Shapiro et al, 2002). Such shieldingmay well have delayed the global reionization process.

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2.4 Optimizing LOFAR for Studying ReionizationLOFAR will detect the spectral signature emitted by neutral hydrogen in the period before fullre-ionization. To optimize the design of LOFAR for the study of this neutral hydrogen, it isimportant to discuss the properties of the expected signal.

2.4.1 IntensityThe signal is expected to be similar in all directions, i.e. it is a global signal. The cosmicbaryon density and the scale factor of the Universe are now sufficiently well established thatthe maximum strength of the HI signal can be predicted to within about 50%. If all neutralhydrogen at high redshifts was distributed uniformly and if the epoch of reionizationcorresponded to z ~ 8.5, the global spectral emission signal at ~ 150 MHz should have abrightness temperature of ~10-20 mK. This assumes the gas emits the 21cm line, thatrequires the spin temperature of the gas to be above that of the CMB, or about 2.73 x (1+z) ~25 K. This signal can easily be detected by LOFAR after an integration of a few hundredhours (e.g. see Table 1.1).

Besides a global spectral feature due to emission, in some regions a radio absorption signalmight also be detected immediately prior to full reionization. When the first ionizing sourcesappeared, before (e.g. stars, quasars or collapsed mini-halos), the hydrogen spin temperaturedecoupled from the emission temperature of the cosmic microwave background (CMB)(Madau et al, 1997; Meiksin, 1999) and may have approached the kinetic temperature of thegas. If such a source was sufficient to heat, but not ionize the gas, the intergalactic mediumwould acquire a spin temperature below that of the CMB temperature. The neutral gas mightthen be seen in absorption against the CMB. The expected signal may well exceed theintensity of the emission signal (e.g. Tozzi et al, 2000),

Figure 2.3 Cartoon of the expected brightness temperature of the cosmic background in the vicinity ofthe hydrogen reionization edge as a function of observing frequency. Three cases are shown for the HIstep, corresponding to different models of how the process took place. All three produce an effect that iscapable of being detected by LOFAR (from Shaver et al.1999).

2.4.2 Expected Angular ScalesPrior to full re-ionization the intergalactic medium was a mixture of neutral, partially ionized,and fully ionized structures. A patchwork of neutral hydrogen emission, and possiblyabsorption against the cosmic background radiation will result in fluctuating sky brightnesslevels with structures up to a degree in size. Simulations suggest that the brightnesstemperature will decrease when the angular scale increases (cf Tozzi et al, 2000).

The low-surface brightness HI signal itself can only be detected by smoothing the data to aresolution of at least a few arcmin. This means that only the inner part of the LOFAR array,out to baselines of a few km, will be effective in mapping the HI signal.

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2.4.3 Implications for LOFAR

LOFAR will be equipped with dipoles optimized for the middle of the 110-220 MHz band. At ~150 MHz LOFAR would be sensitive to HI at z ~ 8.5 which is about in the middle of thefavoured redshift range for reionization.

The size of the virtual core is optimized for detecting a signal from structures of 10’ to 15’ at ~150 MHz, i.e. the expected global signal from the first stages of reionization.

Because the epoch of reionization, and therefore the frequency of the spectral signal, isuncertain a very wide backend, at least 32 MHz and preferably 64 MHz, will be required. Theadopted correlator design should cover such a wide band and allow a finely spaced grid offrequencies to execute the search.

The spectral resolution and instantaneous frequency range is determined by the number ofavailable frequency channels. Taking the strawman design of 4096 frequency channels overa frequency range of ??, an instantaneous frequency resolution of ?? will be attained foreach of the ?? beams. This is well matched to the expected line profile expected from theglobal signal.

Although the collecting area and sensitivity of LOFAR is more than sufficient to detect theexpected reionization signal, need to detect and the reionization experiment imposesstringent requirements on dynamic range and calibration. An expected global reionizationsignal of ~15 mK must be detected within a total noise temperature for the virtual core due tothe 150MHz sky and the system of 0.5K (see Table 1.1), i.e. a dynamic range of 50.

Calibration of the faint expected signal of reionization dictates a telescope with a substantialcollecting area (cf. Shaver et al., 1999) and having long baselines (hundreds of kilometers).Collecting area and resolution are needed to deal with a crucial aspect of this experiment,namely to identify, model, and remove the foreground emission that will contaminate thereionization signal. One obvious contaminant is the population of discrete radio sources. Atthe flux density levels that LOFAR can reach within acceptable integration times (to noiselevels well below 10 µJy at 150 MHz) starburst galaxies will comprise the bulk of the radiosource population. The long baselines of LOFAR (> 100 km) are crucial for isolating andspectrally characterizing these discrete sources, so that they can be removed from thevisibility data on the shorter baselines.

A second even more important contaminant is the diffuse nonthermal galactic foregroundemission which is responsible for the bulk of radio noise from the sky at frequencies at whichLOFAR will operate. Fortunately, this diffuse emission has very little structure on the 5'-60'angular scales at which the signals will be sought. Faint galactic foreground fluctuations thatdo exist should have no spectral features, allowing them to be disentangled from thereionization signal.

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Figure 2.4 Comparison of the predicted brightness fluctuations as a function of angular scale with thesensitivity attainable after integrations of 100 and 1000 hrs with a central aperture of LOFAR. Thesimulated fluctuation spectra show the amplitude of the 3 sigma peaks in sky brightness as computedby Tozzi et al (1999), for two different cosmological models. The sensitivity lines represent a confidencelevel of 5 times the noise level attained after the indicated integration time. The flat portion of thesensitivity curves at the right side of the plot indicate the response of a fully filled aperture of diameterequal 300m to brightness fluctuation on angular scales greater than 22 arcminutes. Dispersing the samenumber of elements over a wider area to obtain better angular resolution causes the sensitivity tosurface brightess to decrease, as denoted by thediagonal line, so that once the aperture is diluted over a 1.5 km diameter area, the instrument shouldstill detect the background peaks at 5 times the noise level on angular scales of 6 arcminutes after 1000hours of integration.

Any area in the galactic halo, where the foreground noise is minimal, can be selected tosearch for the E-o-R signal. Initially several suitable areas, optimized for ionospheric stability(long night-time integrations) and minimum dynamic range requirements (far from strongconfusing radio sources).

Long integration times, approaching weeks or more, will be required and devoted to thisimportant experiment. LOFAR's multi-beaming capability will enable the simultaneousimaging of large areas of sky, increasing the effective integration times. The output of E-o-Rmapping experiments will be a large set of narrow-band images over a wide area of the skyeach encompassing a few hundred square degree of solid angle, over a wide frequencyrange. These 'image cubes' will be analyzed at various spectral and angular resolutions viastandard power spectrum techniques, similar to those used in experiments that studiedfluctuations in the cosmic microwave background (reference). The results will be comparedwith theoretical simulations for a range of cosmological models and sources of re-ionization.

2.5 Answers from LOFAR to Key QuestionsLOFAR will provide a unique tool for investigating the dark ages of the Universe and theepoch of reionization. It will provide answers for the fundamental questions posed in Section2.1.

By detecting and measuring the frequency of the global spectral emission signal from neutralhydrogen just prior to reionization, LOFAR will confirm that reionization occurred in thepresently favored redshift range (10 > z > 7) and accurately determine the epoch ofreionization (Question 1).

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By measuring the width of the global spectral emission feature and the amplitude of spatialfluctuations in the signal, LOFAR will determine how gradually the reionization of the Universeoccurred (Question 2).

By measuring the amplitude and scale of the emission fluctuations and by searching for localabsorption features, LOFAR will constrain the sources responsible for the reionization and thescale of structures that formed the first galaxies (Question 3).

If no signal from the reionization is detected by LOFAR, this in itself will be a fundamentalresult. It will push the epoch at which reionization occurred to well beyond a redshift of z ~ 10and mean that present views about the early evolution of the Universe will need to be revisedradically.

References

Abel et al, Science (2002)Barkana and Loeb, (2000)Barkema and Loeb 2001, Physics Reports 349, 125Loeb and Barkema, 2001, Ann Rev. Astr. Astrophys., 39, 19.Barkana and Loeb 2002Briggs, de Bruyn, Vermeulen, A&A (2001)De Bernardis et al., Nature 404, 995 (2000)Fan et al. Ap.J. (astro-ph 0111184)Gnedin & Ostriker Ap. J. 486, 581 (1997)Gnedin, N. Ap.J., 542, 535-541 (2000)Gnedin, 2001Hui L. et alKamionkowski & Liddle Phys.Rev.Lett. 84, 4525-4528 (2000)Madau, P., Meiksin, A. & Rees, M.J.: Ap.J. 475, 429-444 (1997)Madau, P. 2000Meiksin, A.: In: "Perspectives on radio astronomy: Science with Large Antenna Arrays",published by ASTRON, Ed. M. van Haarlem (1999)Rees, M.J.: In "Perspectives ...", (199)Shaver et al. A&A 345, 380 (1999)Shaver, P. and de Bruyn, A.G.: In "Perspectives ...." (1999)Spergel & Steinhardt: Phys.Rev.Lett. 84, 3760-3763 (2000)Tegmark & Zaldarriaga astro-ph/0004393Tozzi,P., Madau,P., Meiksin,A. & Rees, M.J. Ap J. 528, 597 (2000)Zheng et al. astro-ph/0005247

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3 FORMATION AND EVOLUTION OF GALAXIES,CLUSTERS AND ACTIVE NUCLEI

3.1 Introduction and Key QuestionsOne of the most intriguing problems in modern astrophysics concerns the processes by whichgalaxies and groups of galaxies formed and emerged, from the extremely smooth Universeindicated by the microwave remnant of the original Big Bang (e.g. Lahav et al. 2000 andreferences therein). A related problem concerns the formation of the massive black holes,believed to be located at the centres of most galaxies and to power the enormously energeticexplosive events that are observed as quasars and radio galaxies.

Although there are various scenarios for the development of structure in the Universe, theepoch and mechanism of the formation of galaxies, quasars and clusters are still open. Someimportant questions are:

1. When and how did the first galaxies form?2. When and how did the first active galactic nuclei (massive black holes) form?3. When did the first large-scale structure form and how did the galaxies and intergalactic

gas evolve into rich clusters that we observe in the present Universe?4. What is the relative time sequence for the emergence of galaxies, massive black holes

and proto-clusters? In particular, what is the star formation history of the earliest knowngalaxies and how is this related to the birth of massive nuclear black holes and theformation of galaxy clusters?

The metamorphosis that resulted in the changes mentioned above occurred when theUniverse was between 5 and 30% of its present age, corresponding to the redshift range 7 >z > 1.5. Because the space density of luminous quasars and radio galaxies was severalorders of magnitude larger than now, this period of the Universe is often termed the “quasarera”. During the quasar era the global star formation rate was a factor of 4 - 10 greater than atpresent, massive elliptical galaxies formed and clustering of galaxies began to be formed outof the primeval gas (e.g. Kauffmann et al 1996).

There are three classes of targets in this quasar era that will be observed by LOFAR with thegoal of investigating some of the above questions. These key types of objects are (i) luminousradio sources, produced by black holes in the nuclei of massive forming galaxies, (ii)"starburst" galaxies, i.e. infant galaxies observed to be undergoing a vigorous episode of starformation and (iii) diffuse radio emission that can be used to probe intergalactic gas.

3.2 Distant Radio Galaxies: Probes of Massive Galaxy andCluster Formation

Half a century ago, the discovery of galaxies with radio luminosities of up to 5 orders ofmagnitude greater than our own galaxy inaugurated a new era in observational cosmology.Distant radio galaxies are still unique cosmic probes (e.g. Miley 1999, 2000). Because of theirhuge optical and infrared luminosities and their lumpy appearances they are believed to bemassive galaxies undergoing formation as demonstrated in Fig. 3.1 (e.g. Röttgering and Miley1996, Pentericci 1999, Rengelink 1999, Röttgering et al 1999).

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Figure 3.1 Distant radio galaxies are massive galaxies undergoing formation. This picture taken with theHubble Space Telescope shows that the distant radio galaxy 1138–262 (z=2.2) is extremely clumpy.The structure bears a remarkable resemblance to the predicted images from computer models depictingthe birth of massive galaxies in clusters, through the merging together of large number of small galaxies(from Pentericci et al 1999). LOFAR will locate and study the most distant forming massive galaxies.

Recent Dutch work has shown that luminous distant radio galaxies tend to be located inregions of significant galaxy overdensity, that probably mark the early stages of rich clustersof galaxies (Kurk et al 2000, Pentericci et al 2000). Observations of the environment of onesuch radio galaxy has recently revealed the most distant grouping of galaxies known, apresumed proto-cluster of >20 galaxies at a redshift of z ~ 4.1, corresponding to a distance of~13 billion light years, or ~10% of the present age of the Universe (Venemans et al 2002).

Figure 3.2 The most distant known group of galaxies discovered with the VLT in Chile (from Venemanset al. 2002). LOFAR will pinpoint the most distant forming proto-clusters. (Left) Spatial distribution of 20Ly_ emitting galaxies located in a forming cluster associated with the powerful radio galaxy 1338-193 atz ~ 4.1. (Right) Spectra of the 20 Lyα emitting galaxies in the proto-cluster.

Luminous radio galaxies are therefore excellent laboratories for investigating the birth of thefirst massive galaxies and the formation of the first massive black holes. In addition,they signpost the earliest stages in the formation of rich galaxy clusters.

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LOFAR will detect luminous radio galaxies out to unprecedented distances. The mostefficient method for finding distant radio galaxies was developed in the Netherlands and usesan empirical correlation between radio spectral steepness and distance (Blumenthal andMiley 1979,Tielens et al 1979, Röttgering 1993, van Ojik 1995, de Breuck 1999). Because thesynchrotron-emitting electrons age and lose energy, the nonthermal radio spectrum steepensat frequencies greater than ~ 1 GHz. For distant objects this "break frequency" is redshifted tolonger wavelengths in the observer's frame. Measurement of spectra at the lowestfrequencies (<100 MHz) is needed to filter out radio sources with steep spectra at largedistances.

Figure 3.3 Pinpointing the most distant active galaxies using their steep radio spectra.(Left) Radio spectral index plotted against redshift (distance) for various samples of radio sourcesillustrating that the most distant radio sources tend to have extremely steep nonthermal spectra.Concentrating on objects with the steepest radio spectra is therefore a highly effective technique forfinding the most distant radio galaxies (Blumenthal & Miley 1979).(Right) An optical spectrum of TN J0924-2201, taken with the Keck telescope in Hawaii showing theLyman α spectral line of hydrogen (van Breugel et al. 1999). This object has a redshift of z ~ 5.1 and isthe most distant radio galaxy presently known. It is located at a distance of ~ 13.5 billion light years,corresponding to an epoch when the Universe less than 10% of its present age. By measuring suchspectra at lower frequencies, LOFAR will extend such studies to larger distances.

Large-sky LOFAR surveys at 2 frequencies (e.g. 3000 sq. degrees at 30 MHz and 120 MHz)will easily detect and determine radio spectra for more than 10 million radio sources with fluxdensities larger than 1 mJy at 30 MHz (see Table 1.2). Simple extrapolation of spectra andredshift statistics from existing radio surveys (e.g. Jarvis et al. 2001) shows that the LOFARsurveys will contain more than 1000 radio galaxies with extremely steep radio spectra,located at distances larger than the most distant radio galaxy presently known (Fig. 3.4).

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Figure 3.4 Redshift distribution expected for sources with a spectral index steeper thanα = −1.3 detected in a 3000 sq. degree LOFAR survey at 30 MHz with a flux density limit of 1 mJy. Thedistribution was calculated by extrapolation from data on samples of very bright radio sources compiledby Jarvis et al (2001), under the assumption of a constant comoving source density.

The objects with extreme spectra from the LOFAR survey will be unique targets for followingup with large optical and infrared telescopes. Once their distance (redshift) has beenmeasured, optical and infrared diagnostics will be used to compare with models andinvestigate the processes by which massive galaxies and clusters form in the early Universe.Besides being able to pinpoint the most distant forming massive galaxies, LOFAR will be animportant tool for studying them. Absorption of continuum emission by neutral hydrogen at1420 MHz is a well-known technique for probing gas at high redshifts. By using the ultra-steep spectrum technique to pinpoint suitable continuum "background" targets and byproviding a sensitive (multi-beaming) spectrograph at frequencies < 240 MHz, LOFAR willextend the HI absorption technique to the redshifts > 5. In this way LOFAR can studyneutral hydrogen in the first clumpy proto-galaxies, predicted by hierarchical models ofgalaxy formation.

3.3 Distant Starburst Galaxies: Probes of Galaxy Evolution.A crucial stage in the emergence of galaxies is the epoch at which the first stars formed.Galaxies where large numbers of stars are being produced are important laboratories forstudying galaxy formation. The IRAS satellite advanced the knowledge of such objectssubstantially. One of the most important results of IRAS was the discovery of a class ofgalaxies that are exceedingly bright in the infrared (Soifer et al. 1984). The excess infraredemission is known to be due to radiation by dust produced during enormous bursts of starformation.

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Figure 3.5 Radio emission from M82, the best studied "local" starburst galaxy (from Muxlow et al1995). This galaxy is forming new stars at a rate of about 10 solar masses per year. The many clumpsare due to the remnants of exploding stars ("supernovae”). LOFAR will locate and study anunprecedented number of distant starburst galaxies.

One of the most remarkable properties of galaxies is the existence of a very sharp relationbetween the infrared emission (thermal) and the radio emission (synchrotron) emission.Although van der Kruit (1971) had pointed out the existence of such a relation for nearbygalaxies, a decade later data from the IRAS satellite showed that this correlation extends overseveral orders of magnitude of luminosity, from nearby spiral galaxies to luminous IRASstarburst galaxies (See Figure 3.6 and e.g. de Jong et al. 1984; Sanders & Mirabel, 1985).The infrared-radio relation is explained as a direct consequence of star formation. Theinfrared emission is produced by warm dust, heated by energetic photons produced by youngstars, while the radio emission is due to synchrotron emission produced by the interaction ofsupernovae with the magnetic field of the galaxy.

Figure 3.6 The tight correlation between the 1.4 GHz radio luminosity and the 60 micron IRAS infraredluminosity as observed for a sample of nearby galaxies (from Condon 1991). The infrared-radio relationis explained as a direct consequence of star formation. The infrared emission comes from warm dust,heated by energetic photons produced by young stars, while the radio emission is due to synchrotronemission produced by the interaction of supernovae with the magnetic field of the galaxy. LOFAR willextend such studies to the era of the Universe when galaxy formation was rampant.

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Figure 3.7 The spectral energy distribution of the well-known nearby starburst galaxy M82. The spectralshape is characteristic for all starburst galaxies. The spectrum is a composite one that arises from twomain components: synchrotron emission from supernova remnants (solid line) and thermal emissionfrom dust heated by young stars (dotted line). The dot-dashed line is the predicted spectral componentdue to free-free emission.

The infrared-radio relation is the tightest known relationship involving the global properties ofgalaxies. It contains important information about the distances of star-forming galaxies andabout the processes responsible for producing the stars.

For high redshift objects, the spectrum (see Fig. 3.7) will be shifted redwards and observed atlonger wavelengths. Because of the characteristic and constant rest-frame spectrum, theradio/sub-mm continuum flux density ratio provides a sensitive redshift indicator for starbustgalaxies (cf. Carilli and Yun, 1999). Using a combination of the radio fluxes with data fromfar-IR and millimeter surveys, LOFAR will measure the distances of an unprecedentednumbers of starburst galaxies.

The radio flux densities measured by LOFAR will provide an estimate for the starformation rate of starburst galaxies independent of those derived from thermal radiationfrom stars, at optical, infrared and sub-millimetre wavelengths. These (thermal)measurements of the star formation rate show that star formation increases dramatically withincreasing distance. Because radio measurements are insensitive to dust obscuration, theyare needed in order that a complete census of the cosmic star-formation history can be made.

LOFAR will also search for evolution in the infrared - radio relation. Although changes inthe balance of thermal to nonthermal radiation as a function of redshift would complicate theuse of this relationship as a distance indicator, it would also imply that processes and/ortimescales of star formation in galaxies were different in the early Universe than from now.Unraveling such effects would be of fundamental importance in understanding galaxyevolution.

The depth of the LOFAR surveys, the excellent angular resolution and above all the largesimultaneous sky coverage will make LOFAR a powerful tool for studying large numbers ofstarburst galaxies out to the redshifts at which the bulk of galaxy formation is believed tooccur. The "Rosetta stones" of nearby star-forming galaxies are M82 (see Fig. 3.5 and 3.7)which has a star forming rate of 10 solar masses per year and the "ultra-luminous infraredgalaxy" Arp 220 (z=0.018) which forms stars at a rate of 250 solar masses per year. LOFARoperating at ~200 MHz would be able to detect M 82 out to a redshift of 1.1 and Arp 220 out

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to z=3.3 (e.g. Garrett 2002). The vast majority of radio sources in LOFAR surveys near 200MHz will be distant starburst galaxies. The number of starburst galaxies detected in a 3-yearsurvey with LOFAR should be about 10 million.

To carry out such studies, LOFAR surveys at ~200 MHz will be complemented by a newgeneration of optical and infrared surveys that are already being conducted and will be furtherdeveloped during the next decade, such as Sloan, SIRTF, OMEGACAM on the VST andVISTA. Additional data from radio surveys at high frequencies will provide information aboutthe radio morphologies and spectra, allowing the (more compact) starburst galaxies to be pre-selected and providing lists of targets whose redshifts will be measured using multi-objectspectroscopy. Such multi-object spectrographs will be capable of taking spectra of manyhundreds of galaxies simultaneously. LOFAR will thereby produce large numbers of starburstgalaxies that will be important objects for further study by the Atacama Large Millimeter Array(ALMA) and the Next Generation Space Telescope (NGST).

3.4 Diffuse Radio Sources: Probes of Intergalactic GasEvolution

Extragalactic radio sources have long been used as probes of the surrounding intergalacticgas (e.g. Miley 1980). The radio sources are shaped by pressure from the gas in which theyare embedded (see Fig 3.8). In addition, shocks in this gas (e.g. at the boundaries of galaxiesor clusters) can reaccelerate the older electrons and modifying the radio spectra. In clusters,the motion of the galaxies through the gas during the lifetime of the sources can radicallydistort the shapes of the radio sources, resulting in long tails of trails of radio emission (Mileyet al. 1972). These radio sources provide a "fossil" record. Under various assumptions, thehistory of the nuclear activity and properties of the surrounding gas can be disentangled (e.g.Ensslin et al. 1998, 2000).

Figure 3.8 Examples of cluster radio sources showing interaction with the intra-cluster gas.Small panels - left. Radio images of 4 diffuse radio sources in nearby clusters showing the filamentarystructure of the synchrotron emitting plasma shaped by shocks in the cluster gas (Slee et al 2001).Larger panel - right. Simulation of interaction between a cluster radio source and the intraclustermedium. This simulation takes account of the appropriate gas dynamics and magnetic fieldconfiguration during the evolution of shocks in the cluster gas (Ensslin and Brüggen 2002).The large numbers of cluster radio halos that will be observable with LOFAR will be used to probeevolution in the intra-cluster gas due to e.g. merger-induced shocks out to a redshift of z ~1.

The low frequencies at which LOFAR will operate are optimized for detecting the oldest radiosources with steep spectra and thereby for studying such fossils. The ages of thesynchrotron-radiating electrons are at most 100 million years, i.e. small compared with theage of the Universe (~1.5 billion years). There is increasing evidence that radio galaxies gothrough recurrent episodes of nuclear activity. A study of the statistics of the oldest (steepestspectra) radio sources as a function of redshift will provide new information about the duty-cycle of nuclear activity.

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The excellent sensitivity for steep radio spectra and diffuse features uniquely equips LOFARfor studying such radio fossils and using them to probe intergalactic gas out to highredshifts. The two most important classes of targets for such studies, cluster radio halos andgiant radio sources, probe gas in what are likely to be the densest and sparsest regions of theUniverse.

3.4.1 Cluster Radio Halos: Tracing Over-dense Regions.An important but relatively neglected building block of galaxy clusters are the extendeddiffuse halos of radio emission, sometimes found to inhabit the centres of rich clusters. Thesehalos have typical sizes of ~1 Mpc and steep radio spectra (α< -1), well suited toobservations with LOFAR. The halo synchrotron emission provides diagnostics for studyingthe magnetic field and plasma distribution within clusters, important inputs to models ofcluster evolution.

Prime examples of known cluster halos are the Coma cluster (e.g. Wilson, 1970; Jaffe, 1977),Abell 2256 (e.g. Bridle et al., 1979; Röttgering et al 1994) and A3667 (Röttgering et al 1997;see Fig 3.9). Clusters found to contain radio halos appear to have large X-ray luminositiesand galaxy velocity dispersions (e.g. Hanish 1982), both characteristics of dynamic activitythat would be expected if the clusters are composed of merging sub-clusters as predicted bysome cluster evolution models (Röttgering et al. 1994 and references therein).

Figure 3.9 Cluster radio sources as tracers of turbulence in intracluster gas.Radio and X-ray observations of the rich galaxy cluster Abell 3667 (Röttgering et al 1997, Johnsten et alin prep.). The X-ray emission (data from the XMM satellite shown with false colours) is a tracer of thehot cluster gas. The radio emission (contours from the MOST radio telescope) traces shocks andmagnetic field structure in the cluster gas. The radio arcs observed in the outer region of the cluster arepresumed due to turbulence and shocks caused by the merging of 2 sub-clusters to form Abell 3667.The shocks reaccelerate remnant relativistic plasma previously ejected by cluster galaxies to producethe arc-like radio structures. LOFAR will extend such studies to high redshifts.

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Several models have sought to explain the origin of the relativistic electrons emitting the haloradio radiation. These include (i) superposition of large numbers of relic radio sources (Harrisand Miley 1978), (ii) galactic wakes (Roland 1981), (iii) intra-cluster turbulence and shocksinduced by small clusters falling into the large potential well of the main cluster (Miniati et al2001 and references therein) and (iv) charged pion decay after hadronic interactions with thethermal hot cluster gas (Dolag and Ensslin 2000, and others). Since these models predictdifferent radio morphologies, magnetic field configurations, and spatial variations in radiospectra, LOFAR observations complemented by X-ray images of the hot gas (e.g. XMM andChandra) can help distinguish between these models.

Because of their diffuseness, low luminosities, and steep-spectra, radio halo sources aredifficult to detect with conventional facilities, such as Westerbork (Kempner and Sarazin2001). However, their properties are well matched to LOFAR. The number of cluster halosthat can be detected with LOFAR can be estimated using a simple model proposed byEnsslin and Röttgering (2002). This model takes into account the locally observed fraction ofcluster radio halos, the observed relation between radio and X-ray luminosity, and a Press-Schecter description of the merging rate of massive clusters as a function of redshift. ALOFAR survey at ~120MHz, covering half the sky to a 5-sigma flux limit of 0.1 mJy (1 hourper pointing) is feasible on the time scale of a year and would detect ~1000 halos at the 10sigma level, of which 25 % are expected to be at redshifts larger than z ~ 0.3.

A more direct strategy to find cluster radio halos would be to use the large future clustercatalogues as a target list for deep integrations with LOFAR at frequencies ~100 - 200 MHz.New cluster surveys are being made with the XMM X-ray telescope, the Planck satellite andthe Sloan Digital Sky Survey should catalogue as many as 500,000 new clusters (e.g. Pierreet al 2001, Barthelmann and White 2002). LOFAR has sufficient sensitivity to detect existingradio halos in all these clusters.

Observations of the morphologies of large numbers of such distant radio halos will study theeffects of cluster formation and evolution on the cluster gas, e.g. due to shock wavesproduced by the cluster mergers. If halo sources are indeed associated with the aggregationof sub-clusters, the frequency of their occurrence might well be much higher in more distantproto-clusters, where merging is predicted to be rampant. By measuring the statistics ofcluster halos and their properties as a function of redshift, LOFAR will therefore constrainmodels of cluster formation and evolution.

A fascinating effect related to the diffuse cluster radio sources that may well also beobservable with LOFAR is emission from the large Thompson scattering halos that areexpected to occur around some clusters. Radiation from a bright radio source near the clustercentre should undergo Thomson scattering by the intra-cluster medium that will result in ascattered halo on a much larger scale. The scattering process leads to a significant andcharacteristic polarization pattern allowing it, in principle, to be separated from any intrinsichalo emission (Sunyaev, 1982; Wyse and Sarazin, 1990). The shape and spectrum of theThompson halo carries important information on the time-evolution of the central radiosource, possible beaming of the central source and the properties of the intra-clustermagnetic field. For a 10 Jy source, (not uncommon at 100 MHz for a central cluster source), ascattered halo with flux density of 10 - 100 mJy is expected. This should easily be detectableby LOFAR. Larger scattering halo are expected to be produced around older sources. TheThompson halo around a 107 year-old source will extend out to a projected radius of 3 Mpcand the scattering halo around a 108 year-old source will be 30 Mpc in extent. Such scaleswill be readily mapped in the LOFAR surveys.

3.4.2 Giant radio sources: Tracing under-dense regions.

A small fraction of extragalactic radio sources have sizes that exceed 1 Mpc. These giantradio galaxies (e.g. Willis, Wilson and Strom 1974, Saripalli et al 1986, Subrahmanyan et al1996, Schoenmakers 1999) are amongst the largest known structures in the Universe. Tobecome so large, the giant radio sources must be relatively old (> ~108 y) and be imbedded ina very tenuous intergalactic medium. It is notoriously difficult to obtain good and reliablesamples of such giants since their surface brightness at high radio frequencies is so very low.

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Because of its sensitivity at low frequencies, LOFAR can easily distinguish the very steepspectrum extended emission of giant radio galaxies from confusing background andforeground sources that have flatter spectra. Studies of nearby and distant radio galaxies withLOFAR will pinpoint special regions in the Universe where the intergalactic medium is sparse,i.e. a contrasting extreme from rich clusters. Comparison of the environment of giant radiosources with that of in rich clusters can provide unique information about the effect of large-scale structure on galaxy formation and evolution.Because of their extreme sizes giant radio sources are also an important class of objects fortesting unification scenarios in which the observed properties of different types of activegalaxies and quasars are determined by the orientation of the radio jets with respect to theobserver (e.g. Barthel 1989).

3.5 Gamma Ray Bursters – Prompt Emission and AfterglowsMany of the currently viable models of gamma-ray bursts (GRBs) postulate NS-NS or NS-BHcoalescense as the energy source for the burst. In these models, the possibilities for radioburst generation by neutron star motion through a magnetic field are similar to thosedescribed for the LIGO events above. Another possible source of radio bursts is in themagnetized wind that may flow from the binary system (Usov and Katz 2000). The very largemagnetic field associated with these winds is supported by a surface current at the boundaryof the wind and the ambient medium, and variations in this current may drive coherentemission of low frequency electromagnetic waves. This radiation peaks at frequenciesfundamentally set by the proton gyrofrequency, and for typical burst parameters the peakoccurs around 1 MHz. The high-frequency tail may be detectable by LOFAR; Usov and Katzestimate that flux densities as high as 100 Jy at 30 MHz may be produced.

Radio emission following GRB's has been detected in about 40% of the objects for whichobservations have been possible. This radio emission is usually in the form of “afterglows”,which last for a number of days after the GRB. Shorter lasting (one or two day) “flares” havealso been seen in a few objects. Both radio phenomena show strong absorption at lowfrequencies which indicate that only the very brightest radio events might be detectable withLOFAR. These detections would test flare and afterglow models involving forward andreverse shocks in a frequency range where the models have not yet had to confront data.

The data gathered to date suggest that the gamma rays are beamed and the radio afterglowsare not. Constraints on the size of the opening angle and the observed rate at which GRB'sshow radio afterglows (40%) indicate that for every gamma-ray burst beamed in our directionthere should be 40 radio afterglows. Since, these afterglows last for days, the transient surveyarea covered during their time “on” should be at least half the sky, meaning about 20 radioafterglows per day will fall at some time in the LOFAR field of view. The difficulty will be indetecting the afterglows. Absorption greatly reduces the flux at 150 MHz, from about 100microJy for the typical afterglow to about 2 microJy. The reduced flux means that only about0.3% of the volume out to a redshift of 1 (the typical redshift of a GRB) will be surveyed byLOFAR. Assuming a uniform and isotropic distribution of GRB sources, this brings thenumber of detectable afterglows to about 22 per year. These LOFAR-detected afterglowswould provide a valuable sample for GRB afterglow studies.

3.6 Answers from LOFAR to key questionsThe above discussion shows that LOFAR will provide a wealth of information relevant to thefundamental cosmological questions posed in Section 2.1.1. To summarise the most relevantLOFAR observations:

1. LOFAR will detect radio galaxies at larger distances than hitherto possible. This willconstrain the epoch of formation of the first massive black hole formation (Question 2)Study of these LOFAR distant radio galaxies at other wavelengths will provide informationabout the formation of massive galaxies and links between nuclear activity and star formationin galaxies at the earliest epochs (Questions 1, 4).Since distant radio galaxies pinpoint proto-clusters, study of the environment of LOFARdistant galaxies will constrain the formation of clusters at the earliest epochs (Question 3).

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2. With its unprecedented sensitivity to nonthermal radio emission from star formation,LOFAR will detect large numbers of star-forming galaxies in the early Universe. Studies ofthese objects will provide new information on the evolution of star formation at the earliestepochs of the Universe. Comparison of the LOFAR properties of distant field galaxies withgalaxies in groups (e.g. close to distant radio galaxies) will provide unique information aboutgalaxy and cluster evolution (Questions 1, 2, 3, 4).

3. The statistics and properties of the diffuse steep-spectra radio sources in clusters andaround giant radio sources will provide information about the importance of merging galaxysub-clusters as a function of cosmic epoch and constrain hierarchical models for theformation and evolution of clusters (Question 4).

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astronomical satellite (IRAS) catalogs and atlases. Volume 1: Explanatory supplement, Vol. 1Blumenthal G., Miley G., 1979, A&A, 80, 13Bridle A., Fomalont E., Miley G., Valentijn E., 1979, A&A, 80, 201Carilli C. L., Yun M. S., 1999, ApJ, 513, L13Condon J. J., 1992, ARA&A, 30, 575De Breuck C., 1999, Ph.D. thesis, Univ. Leiden, (1999)de Jong T., Clegg P. E., Rowan-Robinson M., Soifer B. T., Habing H. J., Houck J. R., Aumann H. H.,

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4 THE HIGHEST ENERGY PHENOMENA

4.1 LOFAR as a Cosmic Ray DetectorThe acceleration of particles as observed in Cosmic Rays up to a particle energy of 1020.5 eVis one of the outstanding challenges in High Energy Astrophysics. In particular, in the range ofenergies between 1015 - 1020.5 eV, both the acceleration site and the physical process thatcauses particles to gain such extreme energies are unknown. For comparison, the energy ina 1020.5 eV particle is comparable to the kinetic energy in a tennis ball launched by a worldchampion player. The energy density in CR nuclei is an order of magnitude larger than in CRelectrons in the Galaxy. The CR nuclei are responsible for most of the diffuse gamma-raybackground whereas the CR electrons mainly emit synchrotron radiation at radiowavelengths. At energies below 1015 eV, it is now generally agreed on that the accelerationprocess is a universal first order Fermi-type acceleration, so-called diffusive shockacceleration, in strong shocks that occur in galactic Supernova Remnants (SNRs), andpossibly superbubbles and pulsars as well. At these "medium" energies models exist that canreproduce the observed "universal" particle energy spectrum. The theoretical results areconsistent with observations of a few SNRs both at radio and X-ray wavelengths although adetailed agreement between theory and observation is still lacking. It should be noted thatthere may exist an injection problem for electrons which require preheating and are thereforeless easy to accelerate in the standard shock model than atomic nuclei.

At 1020.5 eV, however, the particle cyclotron radius in the galactic magnetic field is muchlarger than the dimensions of the Galaxy, and simply no container inside the Galaxy is knownto exist which can host and accelerate particles up to such extreme energies. Up to now, onlya few tens of particles have been observed at energies above 1020 eV and only a few eventsper square km per century occur above 4.1019 eV. Possible source candidates of theseUHECR (ultra high energy cosmic rays) are the shocks in radio lobes of powerful radiogalaxies, intergalactic shocks (IGSs) created during the epoch of galaxy formation, thesources of Gamma Ray Bursts (GRBs), or, alternatively, the particles are decay products ofsupermassive particles such as topological defects from phase transitions in the earlyuniverse.

It is clear then that the study of the highest-energy Cosmic Rays is one of the mostinteresting problems in High Energy Astrophysics as it relates to such fundamental problemsas the nature of the intergalactic medium, the epoch of structure formation in the universe, theso-called Hypernovae and merging binary neutron stars in GRBs, and/or fundamental highenergy physics in the early universe.

LOFAR will contribute directly to the study of CRs both at medium and at the highestenergies:1. by the efficient detection of coherent radio emission from UHECRs, and the accurate

determination of the directions of their sources which can then be identified. The coherentradio emission in the form of "antenna radiation" and emitted by the thin (less than theradiated wavelength at the low observing frequencies of LOFAR) slab of secondaryparticles in the electromagnetic cascade that is generated when the primary CR travelsthrough the Earth's atmosphere;

2. by active radar echo detection of UHECR-induced extensive air showers in the VHFfrequency range (30 - 100 MHz). This method of detection is based on scattering of ashort radar pulse off the column of ionized air which is produced in an air shower from aprimary CR as it travels through the earth's atmosphere.

4.1.1 Background Information: Cosmic RaysThe term Cosmic Rays (CRs) stands for energetic particles, both atomic nuclei and electronswith energies up to 1020.5 eV. They have been detected directly in our Galaxy, and indirectlyin our Galaxy as well as other galaxies from their electromagnetic signature in gamma raysdown to the radio domain, e.g. in Supernova Remnants (SNRs), radio pulsars, solar flaresand flare stars, protostars, planetary magnetospheres, X-ray binaries, jets in radio galaxiesand quasars, Active Galactic Nuclei (AGN) and Gamma Ray Bursts (GRBs). Their differentialenergy spectrum is observed to follow a (combination of) power-law(s) over a wide range in

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energies between 108 eV and 1020.5 eV with a power-law index between -2 - -3.8 and, for alarge part, with a typical index -2.7 - -2.8, eg. at 20 - 200 GeV an index of -2.71 ± 0.04 forprotons, and of -2.79 ± 0.08 for helium nuclei (Menn et al. 2000). Small changes in slopeappear at "the knee" around 1015.5 eV, the second "knee" at 1017.8 eV, and the ankle at 1019

eV. CRs with energies above the ankle ("the foot") are called UHECR, ultrahigh energycosmic rays (Sakaki et al. 2001; see Nagano & Watson 2000 for a review). Because of theiruniversal occurrence and comparable properties, much effort has gone into identifying auniversal acceleration process. Nowadays, it is believed that diffusive shock acceleration(Axford et al. 1977; Krimsky 1977; Bell 1978), a first-order Fermi-type acceleration process, isthis universal mechanism (for a recent review see Achterberg 2001). It operates in strongcollisonless shocks such as occur in a multitude of explosive objects in the universe - SNRs,superbubbles, radio pulsars, accreting compact objects and black holes in AGN, jets in X-raybinaries and radio galaxies - and produces a differential power law spectrum in energy withpower law index -2, close to and somewhat flatter than is observed, for any shock as long asthe shock is both strong and non-relativistic,.

4.1.1.1 CompositionAs for the composition of CRs, it should be noted that electrons are observed up to 1012 eVonly, and that in our Galaxy the energy density in electrons is only 6 . 103 eV m-3 ascompared to 5 . 105 eV m-3 for atomic nuclei. The total energy density in CRs near the Sun isabout 1 MeV m-3 (number density 10-3 m-3) comparable to that in thermal energy of theGalactic gas, to the energy density of the Galactic magnetic field, and to that in the CosmicMicrowave Background (CMB). Up to the "knee" in the spectrum at 1015 eV the compositionis dominated by protons but at higher energies the composition is essentially unknown. As theatomic number of a primary CR increases, the muon multiplicity in the atmospheric cascadecreated by the primary particle increases. A recent attempt to determine the composition ofprimary particles around the knee in the L3+C(osmics) experiment at CERN through acombination of the muon detector - the most accurate detector of particle momenta in its time- and an Extensive Air Shower Array on top of the roof of the L3 main hall has demonstratedthat either the primary particles all have a high atomic number or that the simulation codes forthe cascade production are not correct at these energies (Wilkens 2002). The latter possibilityis a realistic one as the cross-sections for forward particle production which go into the codesare not accessible to measurements in the lab.

4.1.1.2 TeV Gamma RaysTeV gamma-rays have been detected in a small number of cases. There is a strong interestin detecting these and other kinds of high-energy particles such as high-energy neutrinos.These could either be produced bottom-up by conventional processes or top-down as decayproducts from "exotic" and "relic" supermassive particles, a collective term used fortheoretically proposed particles in extensions of the Standard Model (e.g., "WIMPS" forweakly interacting massive particles, axions, neutralinos) or topological defects (1024 eV)associated with the boundaries of fields which have undergone a phase transition andsymmetry breaking. High-energy primary gamma rays can be efficiently detected from theirsecondary muons which are produced when the gamma-ray interacts in Earth's atmosphere(Alvarez-Muñiz & Halzen 1999). Detection of high-energy neutrinos is important as adiagnostic of the acceleration process, e.g. in AGN radio lobes. Detection of exotic particles isof direct relevance to determine the nature of the dark matter component in the universe.

A first attempt to detect TeV gamma rays from 8 GRBs detected simultaneously withBATSE on board CGRO has been performed with the muon-detector at L3+C at CERN (vanden Akker et al. 2002). Only an upper limit was obtained, and there is a clear need for moresensitive detectors of such high-energy particles because of their fundamental significance.

4.1.1.3 Neutrino OscillationsA handful of TeV gamma ray sources have, at present, been detected beyond doubt.Essentially, two mechanisms are known to produce such gamma-rays: inverse Comptonscattering of high-energy electrons off their own synchrotron radiation, or alternatively, decayof pions produced by energetic protons colliding with thermal gas in the source. Note that,according to standard shock acceleration theory, such highly energetic protons are expectedto be present at the same location as the electrons. To discriminate between both possibilities

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detection of high-energy neutrinos is important (Waxman & Bahcall 2000) as for example willbe possible with AMANDA. The efficiency of neutrino detection in its turn depends on theneutrino rest masses; if non-zero, oscillations between different kinds of neutrinos occur. Oneinput parameter to the solution of the neutrino oscillation problem is the absolute flux ofsecondary muons at sea level. This has been one of the motivations for the application of theL3 detector at CERN for CR experiments in L3+C as described by van Mil (2001). Theabsolute flux of muons at sea level is expected to be determined by the L3+C measurementswith an accuracy of 4 % at an energy of 100 GeV (Petersen 2002).

4.1.1.4 Energy Budget in Radio galaxiesThere are a number of untested predictions of diffusive shock wave acceleration theory, anda number of remaining problems. According to theory electrons are more difficult toaccelerate than ions. This is known as the injection problem. The observed underabundanceof electrons in the galaxy with respect to nuclei is in qualitative agreement with theory, but itshould be noted that if a similar situation applies to AGN and jets of radio galaxies theminimum energy present in accelerated particles in these objects is enhanced dramatiaclly,and can pose a severe problem to the energy budget.

4.1.1.5 LOFAR and Origin of the Highest Energy CRsExisting model computations of shock wave acceleration agree that CRs can be acceleratedin SNRs up to the knee, either in individual remnants or in a concerted way. However, beyondthe knee, the cyclotron radius of a particle becomes too large for trapping in a SNR, andacceleration of particles at these energies must be of extragalactic origin (Norman et al. 1995;Gallant & Achterberg 1999; Bahcall & Waxman 2000) . Possible sites include the shocks atthe termination of jets in radio lobes of galaxies, intragalactic shocks from the epoch of galaxyformation and the environs of GRBs. Alternatively, they could be decay products ofsupermassive particles (Bhattacharjee & Sigl 2000). As the cyclotron radius of a particle ofenergy 1020 eV becomes comparable to the thickness of the Galaxy accurate determination ofthe direction of motion of the particle would allow discrimation between its possible sources.So far, the celestial distribution of observed UHECR is cosnistent with an isotropic distribution(Dova 2001; Takeda et al. 2001). It is here that LOFAR is expected to play a decisive rolebased on the coherent radio pulses that such energetic particles emit as has been proposedfirst by Jelley et al. (1965; see also Smith et al. 1965) and recently by Falcke & Gorham(2001). A primary CR induces a particle cascade in the atmosphere that is aligned along thedirection of motion of the primary particle. A substantial part of the cascade is leptonic andgives rise to various kinds of radiation, such as Cherenkov emission, transition radiation andalso synchrotron emission in the terrestrial magnetosphere. At the high Lorentz factorsconsidered here the cascade is in fact confined to a slab of meters thickness. As a result, atlong wavelengths the particles act coherently in an antenna-like fashion and the emission isenhanced over the incoherent level by a factor N, N being the number of electrons in abunch. It should be noted that it is the net number of particles - the net difference betweennumber of electrons and positrons - which counts (Alvarez-Muñiz et al. 2000; Gorham et al.2000; Saltzberg et al. 2001). From the arrival times and intensities of the radio pulse atvarious antennas of LOPES (and at a later stage LOFAR) the direction of the primary particlecan then be determined with an accuracy of 1 - 2 degrees. This mechanism forms the core ofthe proposal LOPES for a LOFAR Prototype Station by Falcke & Gorham (2001).

A related problem is the cut-off energy of the CR spectrum. One would expect thatphoto-pion production on the CMB would lead to a cut-off at energies of 5.1019 eV, the so-called GZK (Greisen-Zatsepin-Kuzmin) cut-off. The observed particles at higher energiesimply that the source regions are at a distance less than 100 Mpc. Clearly, it will be importantto actually detect a cut-off, and LOFAR is expected to contribute significantly.

4.1.1.6 LOFAR, active radar and UHECR

An UHECR can be detected also by interrogating the ionized air column produced by theUHECR with radar pulses (Gorham 2001). The UHECR creates an air shower as the primaryparticle passes through Earth's atmosphere which is leptonic for a large part, and ionizes theair. The resulting ionized trail can be detected by scattering of an impinging radar pulse. This

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radar technique is expected to be competitive to fluorescence detection at near-UVwavelengths for primary energies above 1018 eV. Meteors of about 1 _g have a comparablekinetic energy and also generate ionization trails which are detected by radar. The maindifference between the two ionization trails is that a meteor travels at a speed of 30 km/s (atleast below 70 km/s) while the trail from an UHECR appears practically instantaneously. Thetechnique is well suited to detect neutrinos above 1019 eV for which air showers developmuch deeper in the atmosphere. A unique signature of their presence would be the detectionof highly-inclined, near-horizontal showers.

References

Achterberg, A.: 2001 IAU Symposium 195, 291Aharonian, F.A., Atoyan, A.M.: 2000 A&A 362, 937Alvarez-Muñiz, J. & Halzen, F.: 1999 ApJ 521, 928Alvarez-Muñiz, J, Vázquez, R.A., Zas, E.: 2000 Phys. Rev. D 62, 619Axford, W.I., Leer, E., Skadron, G.: 1977 EOS 57, 780 (Proc. 15th Int. Cosmic Ray Conf.,Plovdiv)Bahcall, J.N. & Waxman, E.: 2000 ApJ 542, 542Bell, A.R.: 1978 MNRAS 182, 147Bhattacharjee, P. & Sigl, G.: 2000 Phys. Rep. 327,Biermann, P.L. et al.: 2001 A cosmic ray ground array combined with a radio array in theNetherlands and

GermanyBoggs, S.E., Lin, R.P., Slassi-Sennon, S., Coburn, W., Pelling, R.M.: 2000 ApJ 544, 320Breitschwerdt, D., Dogiel, V.A., Völk, H.J., Kassim, N.E., Weiler, K.W.: 2002 A&A 385, 216Dova, M.T.: 2001 astro-ph/0101379Duric, N.: 2000, in Radio Astronomy at Long Wavelengths, eds. R.G. Stone, K.W. Weiler,M.L. Goldstein,

J.-L. Bougeret, Geophysical Monograph 119, American Geophysical Union,Washington, DC, p.277Gallant, Y.A. & Achterberg, A.: 1999 MNRAS 305, L6Gorham, P.W., Saltzberg, D.P., Schoessow, P. et al.: 2000 Phys. Rev. E 62, 8590Jelley, J.V.: 1965, in Proc. of 9th ICRC vol I, 698Kassim, N.E. & Yusef-Zadeh, F.: 2000, in Radio Astronomy at Long Wavelengths, eds. R.G.Stone, K.W.

Weiler, M.L. Goldstein, J.-L. Bougeret, Geophysical Monograph 119, AmericanGeophysical Union, Washington, DC, p.277

Krimsky, G.E.: 1977, Doklady Akad. Nauk. SSR 242, 1306Longair, M.: 1990, in Low Frequency Astrophysics from Space, eds. N.E. Kassim & K.W.Weiler (Berlin:

Springer-Verlag) p. 227Menn, W. et al.: 2000 ApJ 523, 281Nagano, M., Watson, A.A.: 2000 Rev. Mod. Phys. 72, 689Norman, C.A., Melrose, D.B., Achterberg, A.: 1995 ApJ 454, 60Petersen, B.G.: 2002, The Cosmic Ray Induced Muon Spectrum Measured with the L3Detector, PhD

Thesis, University of Nijmegen, in preparationSakaki, N. et al.: 2001 Proc. of ICRC, 333Saltzberg, D., Gorham, P., Walz, D. et al.: Phys. Rev Letters 86, 2802Smith, F.G., Porter, N.A., Jelley, J.V.: 1965 in Proc. of 9th ICRC, vol I, 701Takeda, M. et al.: 2001 Proc. of ICRC, 341van Mil, A.: 2001, Cosmic-Ray Muons in the L3 Detector, PhDThesis, University of NijmegenWaxman, E. & Bahcall, J.N.: 2000 ApJ 541, 707Webber, W.R.: 1990, in Low Frequency Astrophysics from Space, eds. N.E. Kassim & K.W.Weiler (Berlin:

Springer-Verlag) p. 217Webber, W.R.: 1997 Space Sci. Rev. 81, 107Wilkens, H.G.S.: 2002, Multimuon Events at L3+Cosmics, in Proc. of the XVIII ECRS,Moscow

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4.2 Gravitational Waves - LIGO eventsMainly for practical reasons, most existing radio sky surveys have been made at relatively lowfrequencies. From these surveys have come both serendipitous discoveries (for example,pulsars) and the catalogues of sources used for high-resolution mapping at high frequencies.Serendipitous discoveries often provide the greatest opportunities for advances in ourunderstanding of the physics of astronomical objects. LOFAR would be an excellentinstrument for the study of bursting and transient radio sources. The large instantaneous fieldof view means that LOFAR can effectively monitor a large fraction of the sky at all times,averaging on many different time scales. This makes possible for the first time a sensitiveunbiased survey for astronomical transients. The factor of 100 increase in collecting area overpreviously operated instruments and the array's capability for data buffering and rapidelectronic pointing, assisted at the lower frequencies by plasma delays, will also for the firsttime make possible prompt follow-up of transients detected with LOFAR and otherinstruments.

4.2.1 An Unbiased Survey for Transient Astronomical Radio Sources (STARE)The inner compact configuration of LOFAR will consist of 25% of the array's collecting areaarranged in a circle of 2 km diameter. All signals from the individual receptors will be broughtto the central processor, allowing for synthesis mapping of a substantial fraction of the entirefield of view of the receptors (the receptor field of view is an area of about 100 degreesdiameter for the low frequencies, and about 25 degrees diameter for the highfrequencies).These images will be examined in real time for transient events, and averagingwill provide information on a range of time scales. This activity will represent the first unbiasedsurvey for bursting and transient sources carried out with a radio telescope with a largecollecting area and with sophisticated interference mitigation capabilities. Large-area monitorsfor transient sources at other wavelengths in astronomy have been very productive. Forexample, such observations led to the discovery of gamma-ray bursts and of samples ofsupernovae useful for cosmological studies. A low-frequency radio survey is likely to revealprocesses giving rise to coherent emission, and has great discovery potential.

4.2.2 The Physics of Collapse and Explosion

4.2.2.1 Radio SupernovaeRadio studies of SNe have shown that multi-frequency monitoring of the rapid radio turn-on iscritical to determining the early phases of the explosion, the physical mechanisms at work(e.g., SSA vs. f-f absorption) and the structure and density of the final, pre-supernova stellarmass-loss stages. For very rapidly evolving SNe, only at low frequencies does one havesufficient warning to obtain high quality turn-on information, since a transition seen at 100MHz takes ~15 times as long from explosion to peak flux density as it does at 5 GHz. Forexample, SN 1987A reached 5 GHz peak already at age ~1 day and 840 MHz peak at age~3 days, which led to very poor radio turn-on sampling of this once in 400 year event.Follow-up, long-term, frequent (at least initially) monitoring at multiple frequencies is thenneeded to establish the properties of the last periods of stellar mass-loss before explosion. Atcm wavelengths the presupernova mass-loss evolution for periods of ~20,000–30,000 yearsbefore the explosion can be measured. However, only at long wavelengths does the radioemission remain sufficiently bright that one can study the mass-loss history from even earlierepochs.

Unfortunately, due to the competition between rapidly declining optical depth, slowlydecreasing flux density with time, and increasing flux density at lower frequencies due to thenonthermal spectrum, there is a rough equality of peak flux density at all frequencies. Thus,the peak spectral luminosity even at LOFAR frequencies is unlikely to exceed LR ~ 1027

erg s-1 Hz-1 (flux densities of a few mJy for objects at or closer than the Virgo Cluster) exceptfor unusual cases. Also, because of rapid evolution in the early phases, sensitivity of <1 mJy(5 sigma) in ~30 minutes is desirable with resolution of ≤1 arcsec to reduce confusion withbackground emission from the parent galaxy.

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Radio supernovae are also candidates for targeted searches with LOFAR since they areknown to occur in dusty starburst galaxies (exemplified by the enigmatic objects in M82) withno optical counterpart. To find and study such objects one must carry out continuous, or atleast frequent, monitoring of the radio structure of many nearby galaxies to look for theappearance of new radio point sources. Such observations would improve the rather poorlyestablished supernova rates in different galaxy types.

4.2.2.2 Giant PulsesThe strong magnetic fields associated with neutron stars in pulsars are believed to beresponsible for the nonthermal processes that produce the observed radio pulses. In twocases, these pulses are exceedingly strong, and appear as “giant pulses”. Giant pulses havebeen detected from the Crab pulsar and from the millisecond pulsar PSR B1937+21, both inour Galaxy. In the case of the Crab pulsar, a giant pulse occurs on average once every tenseconds. The flux density distribution of the giant pulses has a power-law distribution above200 Jy. Although the lower flux density cutoff has been determined to be about 50 Jy, noupper flux density cutoff has been seen. Individual giant pulses with flux densities exceeding1000 Jy have been detected, and some giant pulses can outshine the entire Crab Nebula.Pulsars emitting giant pulses with these properties could be detected at least to the galaxiesin the Local Group and potentially even further. Indeed, giant pulse-emitting pulsars offer thebest hope for detecting pulsars beyond the Magellanic Clouds.

Detecting more giant pulse-emitting pulsars would have a number of scientific applications.First, detecting a new Galactic pulsar(s) with giant pulses may make the study of such pulseseasier. The strong intensity of the Crab Nebula makes study of the individual “normal” pulses,and some giant pulses, difficult. Second, the two known giant pulse-emitting pulsars are quitedissimilar: The Crab pulsar is a young, strong-field pulsar while PSR B1937+21 is a(presumably) old, recycled, millisecond pulsar. It is not clear to what extent conclusionsdetermined for one can be applied to the other. Third, existing observations suggest that thegiant pulses from the Crab pulsar occur in the same region as the “normal” pulses, but resultfrom a short modulation of the pulse emission process. Additional pulsars would both test thismodel as well as provide additional constraints on the nature of this modulation mechanism.Finally, the dispersion and rotation measures for pulsars in other galaxies would allow us toprobe their interstellar media and potentially place constraints on the intergalactic medium inthe Local Group.

4.2.2.3 LIGO EventsThe detection of gravitational waves will represent one of the most important measurementsin the history of physics. Evidence for the reality of such signals, and of their astrophysicalorigin, may be provided by other instruments operating in coincidence. Evidence forcoincidence is, of course, not necessary to prove the existence of gravitational waves, buthas the potential to strengthen greatly the case for astrophysical gravitational wave sources,and to provide complementary information on the sources.

The most promising sources for the gravitational wave detectors under construction are thecoalescence of two compact objects in a binary system, including those composed of twoneutron stars (NS-NS binaries), a neutron star and a black hole (NS-BH binaries), and twoblack holes (BH-BH binaries). NS-NS binaries have been most extensively studiedtheoretically. Predicted event rates are highly uncertain, but it has been estimated thatcoalescence will occur at a rate of 3 per year (distance / 200 Mpc)3 for the initial LIGOsystem. Predictions for LIGO II are even more uncertain since at the LIGO II levels ofsensitivity NS-BH and BH-BH events are expected to dominate. Current estimates giveLIGO II rates of once per week for NS-BH events and once per hour for BH-BH events. For atypical coalescing system, the time during which gravitational radiation is emitted within thedetector bandwidth is approximately 15 minutes, during which time there are approximately16,000 cycles. Hansen and Lyutikov (2000) have examined the possibility of radio emissionfrom NS-NS coalescence. They model the system as a conducting sphere (one neutron star)moving through an external magnetic field (due to the other neutron star). They computeinduced currents and the acceleration of charged particles drawn off the neutron star, and byassuming the energy of the charged particles is converted to radio waves with the same

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efficiency as in pulsars, they predict a flux density of F_ ≈ 2.1 mJy (_/0.1) ( D/100 Mpc)-2 B152/3

a7-5/2 where _ is the efficiency of energy conversion, D is the distance, B15 is the magnetic field

in units of 1015G, and a7 is the semi-major axis in units of 107 cm. Again, we emphasize thisprediction is highly uncertain. However, it does give a plausible mechanism for the generationof radio waves in NS-NS coalescence.

One would expect any radio emission to be modulated at the orbital period of the NS-NSbinary. This is roughly given by the dynamical time scale of about a millisecond, though thesignal will be chirped as the binary merges. In analogy with pulsars, one would expect theradio emission to be polarized, and the time dependence of the polarization would provideimportant information on the geometry of the system.

4.3 LOFAR as an All-Sky MonitorLOFAR will detect low-frequency radio emission associated with a very broad spectrum ofobjects whose output in this band is variable and transient.

There are two key aspects to LOFAR studies of the variable sky:

• Large field of view -- LOFAR as an all-sky monitor – the low frequency, centralconcentation of dipoles (`virtual core') and multiple beams provide LOFAR will thepotential to monitor two thirds of the sky daily

• Low-frequency emission -- in addition to, and coupled with, the potential of LOFARas an all-sky monitor, is the possibility to detect for the first time variable emission atvery low frequencies, probing the low-energy tail of particle acceleration and/orpreviously unobserved coherent radiation processes which may be the signatures ofexotic phenomena}

In the following we will discuss these two complementary aspects in turn.

The combination of a central concentration of about one quarter of the LOFAR dipoles withina region ~10 km across, combined with the low observing frequency and potential for multiplebeams allows LOFAR to be used as an all-sky monitor (Fig. 4.1).

Figure 4.1 One concept of LOFAR as an all-sky monitor. Four `virtual core' beams observe the zenithas the sky tracks past, monitoring up to two-thirds of the sky daily. A newly-identified event can then belocalised with arcsecond accuracy using the full array.

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4.3.1 Particle acceleration in cosmic explosions

All of the most energetic phenomena known in our Universe, namely Gamma Ray Bursts(GRBs), Supernovae (SNe), accretion onto supermassive black holes in Active GalacticNuclei (AGN) and accretion onto stellar-mass black holes and neutron stars in X-ray Binarysystems (XRBs) result in radio emission. This is a direct consequence of the energy input intothe surrounding medium resulting in a shock which accelerates particles to high enoughenergies (Lorentz factors may exceed 1000) that they emit synchrotron radiation as theyspiral around the magnetic field lines (which are also probably enhanced in the shock region).In many cases (AGN, XRBs, probably also GRBs) this emitting region is associated with acollimated outflow, or `jet'. By the very nature of these explosive events, this energy input andconsequential particle acceleration are variable phenomena -- recurrent in some cases -- andas a result the radio emission from these sources is also variable.

In more detail, all of the aformentioned phenomena produce a phase of particle accelerationfollowed by energy losses which may be energy dependent (`radiative' synchrotron orinverse-Compton losses dominate) or energy-independent (adiabatic expansion). Theseprocesses are convolved with the evolving optical depth to synchrotron self-absorption withinthe source region itself, which is generally high initially at the low frequencies observable withLOFAR. The general pattern of behaviour is remarkably similar for the different kinds ofsources described above, due to the same underlying physics -- see Fig 2. The violent,relativistic phenomena provide are the lighthouses of the Universe and inject a huge amountof energy into the interstellar, and intergalactic medium. For the first time LOFAR as an all-sky monitor will allow us to detect and monitor these events wherever they are in the sky,providing us with an unparalled study of their properties, distributions etc.

Furthermore, LOFAR will be key to providing alerts of these energetic phenomena to otherfacilities. These violent phenomena currently occupy a large fraction of the world's highenergy astrophysics community, utilising e.g. Chandra, XMM-Newton, HST orbitingobservatories and a vast array of ground-based telescopes, to record data while these eventsare occuring. The goals of such observations are to probe the laws of physics under extremesof gravity, pressure and density in ways which can never be achieved in a terrestriallaboratory. However, these observatories, with their smalls fields of view, require triggering bywider-field detectors. Currently the high-energy astrophysics community is supplied in thisrespect only by the soft X-ray all-sky monitor (ASM) onboard RXTE, and the (less successful)instruments onboard HETE. Neither mission is expected to survive throughout the operationallifetimes of the new class of orbiting observatories, removing from them a key aspect of theiroperation. LOFAR will fill this void, providing notice of activity of all these classes of eventsthrough detection of their radio synchrotron emission, and as such will fulfill a key role for thehigh-energy astrophysics community.

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Figure 4.2 (a): Simulation of a single synchrotron `bubble' event -- a generic representation of theprocesses underlying the radio emission from GRBs, SNE, AGN and XRBs. A shock produces a phaseof particle acceleration of finite, typically short, duration. Subsequently the relativistic gas expands until itbecomes optically thin at progressively lower frequencies. In this simulation the timescales are typicalfor GRBs or XRBs -- at GHz frequencies the emission peaks within a few days and begins to decay. AtLOFAR frequencies the source evolution is slower, due to the greater internal optical depth, and thesource peaks about one month after particle injection, but from this point onwards is stronger than athigher frequencies and may remain visible for a year or more subsequently. This model is based on vander Laan (1966).

Figure 4.2 (b, left): Radio observations of the X-ray transient source CI Cam. The time and frequencyevolution of the event is very similar to that predicted from the simple model presented in Fig 4.2(a).Furthermore, note that at the lowest frequencies (in this case 300 MHz) the rising phase of the eventwas detectable within days of the high frequency emission (which was already peaking). An expandingemitting region (right) was found to be associated with this transient event, which was accompanied bya bright X-ray flare and attracted world-wide attention.

How many of these different types of explosive events we expect to detect can beapproximated based on our knowledge of the coupling between higher-energy emission andradio emission, coupled with the statistics of the properties of these sources at e.g. X-rays. Intable 1 we present estimates for the different types of transient object.

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Predicted detection numbers -- transient sources

Class of object Expected rate per yearGRB afterglows (extragalactic) ~20Supernovae (mostly extragalactic) ~ 1Persistent Galactic X-ray binaries ~ 5Transient Galactic X-ray binaries ~10Transient X-ray binaries in M31 ~1/3

In Figure 4.3 we illustrate the observed radio fluxes of persistent and transient neutron-starand black hole XRBs, clearly demonstrating that most will be readily detectable with LOFAR.In fact, we will probe the luminosity function of such sources deeper than has been achievedin X-rays.

Figure 4.3 Radio vs. X-ray emission for persistent and transient neutron star and black hole X-raybinaries, scaled to a distance of 1 kpc and with X-rays corrected for absorption (Gallo & Fender, 2002).All the sources in this figure would be detected with LOFAR in all-sky-monitor mode (admittedly theweaker sources would take up to a week), whereas the current X-ray all sky monitor onboard RXTE hasa sensitivity of only tens of milliCrab - ie. LOFAR will detect more of these sources than has beenpossible in X-rays.}

Detection of radio emission from sources in M31 and the rest of the local group would be anotable `first'. Furthermore, popular models for the `Ultraluminous X-ray sources' in manyexternal galaxies sugegst that they should be significantly beamed towards us (relativisticaberration, see e.g. Koerding, Falcke \& Markoff 2002) in which case we should also detectseveral of these systems, providing important support for this hypothesis.

It should be noted that the variability associated with the stellar-mass events (GRBs, SNe,XRBs) occurs on considerably shorter timescales than that associated with AGN. Thus, whileAGN may prove to constitute the largest class of significantly variable sources observed withLOFAR, they will be easily distinguishable from the other classes of event.

In addition to the transient, outbursting, sources, quasi-persistent emission associated with jetproduction in X-ray binaries will also be monitored. Currently six systems (Cyg X-3, SS 433,GRS 1915+105, LSI +61 303, Cyg X-1 and Sco X-1) are known which are bright enough thatLOFAR will provide {\em daily} radio light curves. A further, fainter, population of possibly

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{\em fifty} black hole and neutron star XRBs, accreting at lower rates, should be detectedwithin one year.

In both transient and persistent studies we will probe the jet formation characteristics of XRBsfar deeper than ever before. Furthermore, scaling the radio and X-ray fluxes from Fender \&Kuuklers (2001), LOFAR will actually probe the population of X-ray transients considerablyfainter than has been possible with X-ray surveys. For the first time we will be discoveringmore of these phenomena at radio wavelengths than in any other band.

LOFAR as an all-sky monitor, in opening up a new window on the universe, will also provideus with the opportunity to observe coherent low-frequency radio emission from exotic sourcesfor the first time. In all likelihood by monitoring the whole sky at radio wavelengths for the firsttime we will discover not only predicted sources of low-frequency radio emission (nextsection), but also previously unsuspected objects and phenomena -- the prospects are trulyexciting.

4.3.2 New emission regimes at LOFAR frequencies

In addition to its capabilities as an all sky monitor, observing at low frequencies opens up thepossibility of exploring the low-energy distribution of particles in the emitting plasma, withimportant consequences for our understanding of mass-flow in these objects. As well as thesynchrotron emission associated with shock-wave phenomena and consequential particleacceleration, there are also mechanisms suggested for the production of transient low-frequency radio emission via coherent radiation processes associated with exotic events. Weconsider these possibilities further below:

4.3.2.1 Low-frequency synchrotron emissionThe synchrotron emission frequency of a relativistic electron in a magnetic field scales asν∝γ2, where γ is the Lorentz factor of the electron. In observing synchrotron emission below100 MHz, we are probing considerably further towards the low-energy tail of the electrondistribution than has been previously possible. This is important for the total mass in theemitting region, as the number of electrons of a given Lorentz factor resulting from shockaccleration can be considered as a power-law, e.g. $N(γ)dγ ∝ γ-p, where p~2. As an example,the number (=mass) of emitting particles responsible for an optically thin synchrotronspectrum between 1-15 GHz (a typical broad observing range) is four times less than thatrequired to explain the same spectrum observed to 100 MHz, and ten times less than thatrequired to explain the spectrum observed to 20 MHz. Quantitatively constraining the massoutflow and comparing with the mass inflow rate is a key question for models of the accretion--outflow coupling in XRBs and AGN.

Furthermore, low-frequency imaging of radio-jet XRBs may be expected, in analogy to AGN,to reveal low-surface brightness radio nebulae powered by the jet, providing an independentmeasure of the energy budget of the outflow and its interaction with the ISM (see Fig 4).

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Figure 4.4 A large-scale (> 5 arcmin) radio nebula powered by an inner relativistic jet from the neutronstar XRB Cir X-1, observed at 1 GHz with the Australia Telescope Compact Array (from Fender 2002).These observations are at 1 GHz; already at 2.3 GHz the nebula is resolved out - at LOFAR frequenciesmany more of these structures should be detected, revealing the interaction of the jet with the ISM, andproviding an independent measure of the mass and energy output from the jet.

4.3.2.2 Coherent processes

Beyond synchrotron emission, coherent radiation processes may be detectable at LOFARfrequencies. Predictions include LIGO Events and prompt emission from GRBs:

The detection of gravitational waves will represent one of the most important measurementsin the history of physics. Evidence for the reality of such signals, and of their astrophysicalorigin, may be provided by other instruments operating in coincidence. Evidence forcoincidence is, of course, not necessary to prove the existence of gravitational waves, buthas the potential to strengthen greatly the case for astrophysical gravitational wave sources,and to provide complementary information on the sources.

The most promising sources for the gravitational wave detectors under construction are thecoalescence of two compact objects in a binary system, including those composed of twoneutron stars (NS-NS binaries), a neutron star and a black hole (NS-BH binaries), and twoblack holes (BH-BH binaries). NS-NS binaries have been most extensively studiedtheoretically. Predicted event rates are highly uncertain, but it has been estimated thatcoalescence will occur at a rate of 3 per year (distance / 200 Mpc)3 for the initial LIGOsystem. Predictions for LIGO II are even more uncertain since at the LIGO II levels ofsensitivity NS-BH and BH-BH events are expected to dominate. Current estimates give LIGOII rates of once per week for NS-BH events and once per hour for BH-BH events. For a typical

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coalescing system, the time during which gravitational radiation is emitted within the detectorbandwidth is approximately 15 minutes, during which time there are approximately 16,000cycles. Hansen and Lyutikov (2000) have examined the possibility of radio emission from NS-NS coalescence. They model the system as a conducting sphere (one neutron star) movingthrough an external magnetic field (due to the other neutron star). They compute inducedcurrents and the acceleration of charged particles drawn off the neutron star, and byassuming the energy of the charged particles is converted to radio waves with the sameefficiency as in pulsars, they predict detectable radio emission at LOFAR frequencies.

One would expect any radio emission to be modulated at the orbital period of the NS-NSbinary. This is roughly given by the dynamical time scale of about a millisecond, though thesignal will be chirped as the binary merges. In analogy with pulsars, one would expect theradio emission to be polarized, and the time dependence of the polarization would provideimportant information on the geometry of the system.

Many of the currently viable models of GRBs postulate NS-NS or NS-BH coalescense as theenergy source for the burst. In these models, the possibilities for radio burst generation byneutron star motion through a magnetic field are similar to those described for the LIGOevents above. Another possible source of radio bursts is in the magnetized wind that mayflow from the binary system (Usov \& Katz 2000). The very large magnetic field associatedwith these winds is supported by a surface current at the boundary of the wind and theambient medium, and variations in this current may drive coherent emission of low frequencyelectromagnetic waves. This radiation peaks at frequencies fundamentally set by the protongyrofrequency, and for typical burst parameters the peak occurs around 1 MHz. The high-frequency tail may be detectable by LOFAR; Usov & Katz estimate that flux densities as highas 100 Jy at 30 MHz may be produced.

References

van der Laan H., 1966, Nature, 211, 1131Fender R., 2002, in prepFender R., Kuulkers E., 2001, 324, 923Gallo E., Fender R., 2002, in prepHansen B.M.S., Lyutikov M., 2001, MNRAS, 322, 695Koerding E., Falcke H., Markoff S., 2002, A\&A, 382, L13Usov V.V., Katz J.I., 2000, A\&A, 364, 655

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5 THE MILKY WAY AND NEIGHBOURING GALAXIES

LOFAR will contribute directly to the study of CRs both at medium and at the highest energiesby detailed imaging of the galactic distribution at low frequencies where the emission isdominated by sunchrotron radiation from CR electrons. These observations will allow adetailed confrontation with theory and permit the ultimate test of the standard model ofdiffusive shock acceleration. Determination of the CR electron distribution will be possible bydetailed investigation of absorption "holes" in the synchrotron radio emission from the Galaxyat low frequencies. Such absorption is imprinted by intervening ionized thermal gas aroundyoung hot stars in the Galaxy (HII regions) with known distances on the radio emission on itsway to Earth. From the amount of absorption the synchrotron emissivities can be derived, andfrom it, given the magnetic field strength, the spatial distribution of the electron component inCRs. These can then be compared directly to the electron distribution obtained from the soft(< 1 MeV) Galactic gamma-ray background which are caused by bremsstrahlung and inversecompton scattering of CR electrons (< 1 GeV). Comparison with the nuclear component inCRs as obtained from the diffuse galactic gamma-ray background above 100 MeV providesan important test of the diffusive shock acceleration mechanism. This technique requiresobservations at low frequencies at high spatial resolution (of 1') and a high sensitivity (downto 0.1 mJy/arcsecond2);

5.1 Testing the Standard Shock Acceleration ModelSNRs, both in our Galaxy and in neighbouring galaxies appear to be the sources of most ofthe accelerated electrons observed in their radio synchrotron emission. This is corroboratedby gamma-ray observations which reveal several strong sources associated with SNRs inour Galaxy and which are caused by CR nuclei (Webber 1997). X-ray spectra of SNRs whichstill have a pulsar inside ("plerions") are compatible with Fermi acceleration at relativisticshocks (Gallant 2002; van der Swaluw 2002). The fact that acceleration models of SNRs canproduce the required number of particles up to an energy of 1015 eV is not a proof that this ishow the Galaxy accelerates particles. A definite test will be to determine the actual spatialdistribution of CR electrons and CR nuclei. In principle this can be done by combining radioobservations from the Galaxy with the galactic diffuse gamma-ray emissions as follows(Longair 1990; Webber 1990; Duric 2000). At the low frequencies observable with LOFAR thegalactic synchrotron emission dominates. Further, HII regions become optically thick below30 MHz and absorption caused by the ionized gas in galactic HII regions with knowndistances shows up in high resolution observations as holes ("swiss cheese" appearance). Adetailed spatial distribution of the synchrotron emissivity as a function of frequency (andpolarization) can then be determined. Together with magnetic field determinations, eg. fromradio pulsar measurements, the spatial distribution of energetic electrons between 200 - 300MeV can then be determined, and compared with electron distributions independentlyobtained from the galactic soft (30 - 800 keV) gamma-ray background which is a combinationof relativistic bremsstrahlung and Inverse Compton scattering of CR electrons below 300 MeV(which cannot be measured near Earth due to solar modulation) off the interstellar radiationfield (Boggs et al. 2000). Further, this CR electron distribution can be compared to the spatialdistribution of energetic protons inferred from the diffuse hard gamma-ray (100 MeV - 100GeV) galactic background which comes from secondary �0 mesons which are produced whenCR protons collide with the ambient interstellar gas (Aharonian & Atoyan 2000). These datathen constitute a strong test on the acceleration models. A similar procedure can be appliedto nearby galaxies. Conversely, the data give a handle on the existence of a galactic wind.For instance, the discrepancy between the radial dependence of Galactic CR nuclei asinferred from EGRET diffuse gamma-rays above 100 MeV and the most likely CR distributionfrom SNRs, superbubbles and pulsars, can be explained by propagation effects in a galacticwind where CR transfer by diffusion is taken over by advection in the wind at a certain heightabove the disk (Breitschwerdt et al. 2002).

Finally, reliable determinations of spatial variations in the spectral index of the radiocontinuum emission in a number of selected well-resolved SNRs at low frequencies such aswill be possible by LOFAR allow testing of the diffusive shock acceleration hypothesis andstudy of the problem of the injection energies (Kassim & Yusef-Zadeh 2000).

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5.2 Thermal and Nonthermal Emission in Nearby Galaxies

Radio continuum emission from galaxies provides significant information on the properties ofthe interstellar medium. At low frequencies radio emission will be dominated by thesynchrotron mechanism. At frequencies between 20 and 300 MHz one predominantlyobserves the old population of relativistic electrons present in the disks and haloes (or thickdisks) of galaxies, i.e. the aged version of the relativistic electrons which were supposedlygenerated by supernovae and associated events (such as the supernova shocks ploughingthrough the interstellar medium).

Imaging the radio emission of galaxy disks at low frequencies will enable us to separatebetter the thermal and non-thermal emission. This is done best by comparing many imagesspanning a large range in frequency rather than images at only a few frequencies as hasbeen done until now. Escpecially high resolution imaging below 300 MHz will provide a muchmore robust determination of the non-thermal component, and by implication the thermalcomponent in higher-frequency images. This is important, because it provides us with one ofthe least biased views of star formation in galaxies. Low-frequency imaging at the same timeprovides detailed information on spectral index variations across galaxies, which in turnfunction as important clues to location-dependent aspects of relativistic electron productionand evolution.

Only a few low-frequency surveys of galaxies presently exist at 80 and 160 MHz (Slee1972a,b; 1977) and at 57.5 MHz (Israel and Mahoney 1990), all with limited resolutions toopoor to resolve any spatial structure. However, they do imply the frequent occurrence ofsignificant spectral flattening also at the low-frequency end, with spectral turnovers in the 100- 1000 MHz range. This flattening may be explained by relativistic electron energy losses orfree-free absorption of synchrotron emission by the ionized interstellar medium. Israel &Mahoney (1990) have shown that in the latter case, galaxies must have a pervasive presenceof well-mixed clumpy thermal/nonthermal gas, with an ionized component at temperatureswell below Te = 103 K. This component might correspond to the ionized fraction of the coldneutral interstellar medium. Alternatively, the observed spectral turnovers could be caused byrelativistic electron energy losses in large-scale galactic winds (Hummel 1991; Pohl et al1991). At least in the case of the Local Group galaxy M33, Israel et al. (1992) found that free-free absorption is the most probable cause of the observed low-frequency spectral flattening.

Either explanation is extremely interesting. If free-free absorption is the dominant process,further low-frequency observations provide one of the few ways, if not the only one, ofstudying a major but hitherto virtually inaccessible component of the interstellar medium. Ifgalactic winds apply, low-frequency observations again provide one of the few ways ofstudying the large-scale energetics and disk-halo interactions of spiral galaxies. Furtherprogress in this field requires accurate determination of integrated galaxy spectra in thefrequency range 20 – 1000 MHz, with good spectral sampling. LOFAR would play theessential role at the lower end of this range. In addition, low-frequency imaging of e.g. edge-on galaxies is also powerful discriminating tool as galactic wind models predict a spectrasteepening away from the galactic plane, whereas the free-free absorption model predictslittle or no spectral change perpendicular to the plane. In the plane itself there will be strongabsorption by ionised gas in HII regions.

ReferencesHummel K., A&A, 251, 442, 1991Israel F.P., Mahoney M.J., 1990 ApJ, 352, 30Israel F.P. et al. 1992, A&A, 261, 47Pohl et al., 1991 A&A, 250, 302Slee O.B., 1972a ApL, 12, 75Slee O.B., 1972b Proc Astr Soc Austral, 2, 159Slee O.B., 1977 Austral. J. Phys. Suppl., 43, 1

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5.3 Supernova Remnants and the Distribution of Ionized Gasin the ISM

The nonthermal spectra and extended morphologies of Galactic SNRs ideally matchLOFAR’s unprecedented, low frequency surface brightness sensitivity. The naturally largefields encompass the largest, individual Galactic SNRs and efficiently survey wholepopulations in nearby galaxies. Continuum spectrum studies are more accurate because ofwide frequency coverage, and thermal absorption and scattering effects can only bemeasured at LOFAR frequencies.

5.4 Discovering New SNRsCatalogs are severely selection effect limited and incomplete at surface brightness levels~10-20 W m-2 Hz sr-1 (Green 1991), which LOFAR will exceed by two orders of magnitude.Catalogs under-sample both older SNRs and bright, compact, young SNRs, required forunderstanding SN/SNR birthrates, statistics, ISM energy input, and for comparison withprogenitor populations. Sensitive (< 1 mJy/beam) LOFAR Galactic plane surveys will easilyreveal many (>100) new SNRs and PSR/SNR associations, critical to studies of the radiolifetime of SNRs and PSR birth kinematics (Frail et al. 1994). LOFAR will also reveal uniquesources such as the SS433 jet system within W50 and as seen in the Galactic center. Recent330 MHz VLA images have shown an unprecedented combination of resolution and surfacebrightness sensitivity and has detected many new exotic sources (Kassim & Frail 1996, Langet al. 1999, LaRosa et al. 2000). However it falls short of detecting emission on scales largeenough to connect with structure seen on single dish maps (Sofue 2000) and linked toGalactic center black hole activity. LOFAR’s dense uv coverage will achieve this and provideequally spectacular inner Galaxy maps in many more directions.

5.4.1 Spectral StudiesSNR radio continuum spectra trace the energy spectra of shock accelerated relativisticelectrons and test predictions of shock acceleration theory, probing the complex modulationof blast-wave physics by the pre-existing environment into which remnants evolve. Theory ispresently very poorly constrained, since models predict varieties of subtle variations whichcurrent observations are too inaccurate to test (Jones et al. 1998). Increasingly believableevidence of spectral variations is emerging, but the poor accuracy of the primitive lowestfrequency data remains the weakest link and guarantees that LOFAR will have a powerfulimpact.

LOFAR will revolutionize the accuracy of SNR spectra where Fermi theory predicts curvatureand strongly constrains magnetic fields. Present spectra are inaccurate at low frequencies,and believable curvature is suggested towards only a few sources (Reynolds & Ellison 1982).Integrated spectra also test Van der Laan theory where controversial bends are linked tocosmic ray gas compression (Green 1990). Spatial spectral variations test theory in greaterdetail, e.g. steep spectrum breakout regions interpreted as weaker shocks (Pineault et al1998). But curved electron spectra, magnetic field gradients, particle acceleration/diffusionand other effects weave a complex relationship which present maps are too crude toconstrain. This is despite the maturity of spectral analysis (Anderson & Rudnick 1993, Zhanget al 1997) utilizing new techniques (e.g. spectral tomography, Katz-Stone et al. 2000) robustagainst calibration errors. But they cannot overcome the poor resolution and sensitivity of thelowest frequency images. LOFAR spectra will distinguish remnant morphologies (e.g.composites and barrels, Dwarakanath 1991, Gaensler, 1998) and reveal the influence ofpulsar winds (Frail et al 1994). A preeminent mystery is the missing Crab shell, demandingexplanation in context of blast wave physics/ISM properties. LOFAR will improve the best limit(Frail et al 1995) by two orders of magnitude.

5.4.2 Absorption & ISM StudiesThe surprising discovery of SNR internal absorption at 74 MHz implies LOFAR will detectthermal material inside numerous remnants. The Cas-A absorption (Error! Referencesource not found.) is only the second detection of unshocked ejecta inside a young SNR

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(Kassim et al 1995). It is expected from theory and provides a crucial probe of reverse shockphysics in young SNRs. The Crab also shows thermal absorption from its foreground thermalfilaments, constraining their location relative to the pulsar powered nonthermal emission(Bietenholz et al 1997). Extending SNR thermal absorption measurements beyond thesepathologically bright sources requires LOFAR.

SNR integrated spectra reveal low frequency turnovers (Kassim 1989) due to extrinsicthermal absorption. This offers a powerful constraint on the distribution of ionized ISM gas,including hard upper limits on the Warm Ionized Medium density (≤ 0.26 cm-3) which can begreatly improved by lower frequency (≤ 50 MHz) LOFAR measurements. The patchyabsorption is consistent with Extended HII Region Envelope absorbers suggested bystimulated low frequency recombination lines (Anantharamaiah 1986) which LOFAR will alsobe uniquely sensitive too. However existing spot frequency data are at primitive resolution(>15’), with the 74 MHz VLA image in Error! Reference source not found. being the firstspatially resolved detection of ISM absorption against a Galactic SNR (Lacey 2001). A >100times more sensitivity, frequency versatile LOFAR will redefine this field of study. For innerGalaxy complexes the contrast between absorption and emission will disentangle the relativesuperposition of HII regions and SNRs as in W30 (Kassim & Weiler 1990) and the Galacticcenter (Pedlar et al 1989), but in most cases is impossible without LOFAR.

Scattering measurements test blast wave physics which predicts turbulence near SNRshocks. These are accessible to LOFAR through measurements of background sources, withbroadband coverage distinguishing between competing absorption effects. VLBI has exploredfor such effects, e.g. the heavily scattered line of sight towards 1849+005 passingcoincidentally ~10’ near SNR G33.6+0.1 (Spangler et al 1986), but connections remainunproven. The myriad, sub-mJy background sources and the λ2 dependence of scattering willallow many more measurements by LOFAR.

5.4.3 Extragalactic SNR StudiesNearby galaxies provide powerful statistical samples of co-distant SNRs (Jones et al 1998).LOFAR will find new SNRs and anchor spectral and morphological identifications made athigher frequencies. Many SNRs show extrinsic turnovers at higher frequencies (≥ 100 MHz)than towards Galactic SNRs. At 408 MHz in M82 this implies ionized gas with emissionmeasures ~106 cm-6 pc, comparable to giant HII regions (Wills et al. 1997). This places thediscrete sources relative to the host galaxy ISM, and measures particle accelerationefficiency, relates cosmic ray origin to specific SNe types, and measures SN rates, starformation, and ISM properties (Duric et al 1995, Jones et al 1998). Lower frequencyobservations are crucially required but without LOFAR have insufficient resolution.

In summary, LOFAR will offer unique insights into SNRs and their interaction with the ISM,with its surface brightness sensitivity and field of view well matched to SNR spectra andmorphology. Intrinsic absorption seen by the 74 MHz VLA imply LOFAR will trace thermalmaterial within many more SNRs, and it will complete SNR catalogs by uncovering many(>100) new remnants and SNR-PSR associations. Its broad-band response and highresolution will empower shock-acceleration studies where sensitive, spatially resolvedspectral index maps trace synchrotron emitting relativistic electron energy spectra.Comparisons with higher frequency images will measure spectral variations at unmatchedaccuracy to test theoretical predictions, and will re-define integrated spectra. LOFAR will be apowerful diagnostic of ISM gas, with SNRs and background sources providing a grid againstwhich to map the ISM through absorption and scattering processes. LOFAR SNR applicationsextend to nearby galaxies, mapping the ionized ISM with respect to the discrete sources.Powerful, co-distant sampled LOFAR SNR images and spectra will provide powerfulconstraints on the star formation history, ISM energetics, and cosmic ray gas properties ofnearby galaxies.

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Duric, N., Gordon, S.M., Goss, W.M., Viallefond, F., & Lacey, C., 1995, ApJ, 445, 173.Dwarakanath, K.S., 1991, JA&A, 12, 199.Frail, D.A., Kassim, N.E., Cornwell, T.J., & Goss, W.M., 1995, ApJ, 454, L129.Frail, D.A., Kassim, N.E., & Weiler, K.W., 1994, ApJ, 107, 1120.Frail, D.A., Goss, W.M., & Whiteoak, J.B.Z., ApJ, 437, 781.Gaensler, B.M., 1998, ApJ, 493, 781.Green, D.A., 1990, AJ, 100, 1928.Green, D.A., 1991, PASP, 103, 209.Jones, T.W., Rudnick, L., et al. 1998, PASP, 110, 125.Kassim, N.E., 1989, ApJ, 347, 915.Kassim, N.E., Perley, R.A., Dwarakanath, K.S., & Erickson, W.C., 1995, ApJ, 455, L59.Kassim, N.E., & Frail, D.A., 1996, MNRAS, 283, L51.Kassim, N.E. & Weiler, K.W., 1990, ApJ, 360, 184.Katz-Stone, D.M., Kassim, N.E., Lazio, T.J.W., & O'Donnell, R., 2000, ApJ, 529, 453.Lacey, C.K., Lazio, T.J.W., Kassim, N.E., & Dyer, K., 2001, ApJ, (in press).Lang, C., Anantharamaiah, K.R., Kassim, N.E., & Lazio, T.J.W., 1999, ApJ, 521, L41.Larosa, T.,N., Kassim, N.E., Lazio, T.,J.,W., & Hyman, S.D., AJ, 119, 207.Pedlar, A., Anantharamaiah, et al. 1989, ApJ, 342, 769.Pineault, S., Landecker, T.L., Swedlyk, C., & Reich, W., 1998, JRASC, 92, 31.Reynolds, S.P. & Ellison, D.C., 1992, 399, L75.Sofue, Y. 2000, ApJ, 540, 224.Spangler, S.R., Mutel, R.L., Benson, J.M., & Cordes, J.M., 1986, ApJ, 301, 312.Wills, K.A., Pedlar, A. Muxlow, T.W.B., & Wilkinson, P.N., MNRAS, 291, 517.Zhang, X., Zheng, Y., Landecker, T. & Higgs, L.A., 1997, A&A, 324, 641.

5.5 H II RegionsObservations at 330 MHz have demonstrated that measurements of optically thick H IIregions can constrain source electron temperatures, emission measures, and filling factors.At lower frequencies these regions appear as cooler regions against a much hotter Galacticbackground, allowing kinematic distance ambiguities to be resolved and the superposition ofthermal and nonthermal sources to be separated along complex lines of sight through theGalaxy. A classic example is the 330 MHz VLA observation that revealed the thermal Galacticcenter source Sgr A West in absorption against the nonthermal Sgr A East supernovaremnant, constraining the superposition of these sources along the most confused Galacticline of sight. Along other lines of sight, kinematic-distance ambiguities resulting from radio-recombination-line measurements can be resolved using the detection, or non-detection, of HII regions in absorption below 100 MHz, because foreground (“near”) H II regions would bemuch more prominent absorption features on low-frequency LOFAR images than distant(“far”) ones.

5.6 Interstellar Propagation EffectsAll Galactic and extragalactic radio sources are observed after their radiation has propagatedthrough the Galactic plasma (Rickett 1990). Variations in the plasma density producerefractive index fluctuations, scaling as ν-2, which in turn scatter the radiation. The magnitudeof radio-wave scattering from the interstellar plasma is strongly direction dependent, but theeffects can remain significant at frequencies as high as 10 GHz or higher. The density(refractive index) microstructure responsible for interstellar scattering occurs on scales oforder 1 AU. The density fluctuations, in turn, are thought to arise from velocity and/ormagnetic field fluctuations. In addition to their corrupting effects, interstellar propagationeffects are a powerful sub-parsec probe of the interstellar plasma, can provide a tracer ofenergy input into the ISM, can serve as a filter to find extremely compact sources, and maybe linked to cosmic ray propagation. The strong frequency dependence of interstellarscattering means that studies of it are optimized with high-resolution, low-frequencyobservations.

LOFAR would prove useful for interstellar scattering studies in a number of respects. Themost recent compilation of scattering observations contained 223 sources, a number that will

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be greatly increased by LOFAR. Not only will LOFAR increase the number of known pulsarsby a large amount, but it will also enable scattering observations in less intense scatteringregions. The vast majority of scattering studies have been conducted using compactextragalactic sources viewed through regions of intense scattering (e.g., Cygnus), where thescattering effects can be detected at frequencies near 1 GHz on baselines of length50–5000 km, typically with VLBI. The use of VLBI has also restricted most scattering studiesto relatively bright sources. Because of the ν2 dependence of scattering and LOFAR'ssensitivity a much larger volume of the Galaxy will be opened for scattering studies.

Of particular interest is the distribution of the scattering material near the Sun and its spatialspectrum. There are conflicting claims for the detection of a signature from the boundary ofthe Local Bubble. The interior of the Local Bubble is a hot, nearly fully ionized gas whereasthe gas outside is cooler and perhaps only partially ionized. At the interface, one might expectsome amount of turbulence and an increased level of scattering. However, the generallyweak level of scattering near the Sun means that a signature of this interface is difficult todetect at frequencies near 1 GHz. The magnitude of scattering observables, e.g., angularbroadening, depends not only upon the total rms electron density fluctuations toward asource, but also upon their distribution along the line of sight. Thus, multi-frequency scatteringobservations of nearby pulsars may resolve the local structure of the interstellar medium.

Current shock-acceleration theories, relevant to the origin of cosmic rays, also suggest thatupstream of a SNR should be an ideal site for the generation of the density fluctuationsresponsible for interstellar scattering. High-frequency searches for the signatures of suchupstream turbulence have a mixed record. The ν2 dependence of interstellar scattering wouldallow much more stringent tests to be applied.

Compact incoherent extragalactic sources should be limited by the inverse Comptoncatastrophe to brightness temperatures of no more than about 1012 K. Nonetheless, varioussources have been identified that have inferred brightness temperatures well in excess of theCompton limit (by factors of 102–103). The primary means for identifying these sources hasbeen via their variability: Low frequency variables (LFVs) are sources that vary on time scalesof order 1 year at frequencies below 1 GHz while intraday variables (IDVs) are sources thatvary on time scales of order hours at frequencies near 5 GHz. Various lines of evidence,including a dependence on Galactic latitude, suggest that the variability is extrinsic in originand results, either partially or entirely, from refractive interstellar scintillation (RISS). Thevariations are caused by AU-scale density fluctuations drifting past the line of sight andfocusing and defocusing the background source. The effective velocity of the densityfluctuations is determined by a combination of the Earth's velocity and motions within theinterstellar medium. Just as in the case of optical scintillation in the Earth's atmosphere (“starstwinkle, planets don't”), in order to display RISS a source must be less than a characteristicsize. The implied diameters of LFV and IDV sources are of order 10 _as to 10 mas, wellbelow can be probed even with current space-based VLBI at the relevant frequencies. ThusRISS, via LFV and IDV, can serve as a probe of the interstellar medium on AU-scales and afilter on source diameter selecting milliarcsecond- and sub-milliarcsecond-diameter sources.

Monitoring programs typically indicate that only a small fraction, ~1%, of sources are compactenough to display LFV. IDV is a more recently recognized phenomenon, but initial monitoringprograms suggest that the proportion of IDV sources might be much higher. Although nodetailed comparison of IDV and LFV has been done, other than the possible proportion ofvariable sources, sources displaying either of the two phenomena have a number ofsimilarities. Most notable is that flat-spectrum sources are most likely to display either IDV orLFV. With notable exceptions, LFV monitoring programs have consisted of measuring the fluxdensity of sources at only a few epochs. Sampling a large number of sources relativelyfrequently might reveal that a much larger fraction of sources really are LFVs. Such amonitoring program would also elucidate motions of the Earth relative to the local interstellarmedium potentially as well as motions within the interstellar medium. An exemplar of this kindof monitoring program is that of Bondi et al. (1994) who were able to determine the Earth'smotion relative to the local interstellar medium by finding annual variations in the level ofvariability. Their analysis relied on only 43 sources; with a sufficiently large number of sourcesone could consider differences in the level of annual variations in different directions, whichwould presumably indicate motions within the medium.

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With its relatively large beams LOFAR would be a powerful monitoring instrument. It is notimplausible that any given region of the sky (and the sources within it) would be observed onaverage once a week. Over the lifetime of the instrument, 5–15 year light curves with nearlyweekly sampling could be produced on a considerable number of sources. Such light curveswould be powerful diagnostics of RISS-induced (and intrinsic) variability. The results of amonitoring program could also be used to define a homogeneous sample from which to probeother aspects of LFV. For example, is there a redshift dependence on the likelihood for asource to display LFV? The presence (or absence) of such a dependence would probe theevolution of radio sources. The long time scales of LFV, as opposed to those of IDV, make itsidentification easier in large samples of sources. In an individual source, IDV can be identifiedrelatively quickly, but with current single-beam, high-frequency instruments monitoring a largenumber of sources is demanding on telescope resources. Conversely multi-beam, high-frequency instruments, such as the proposed Square Kilometer Array, are scheduled tobecome operational around 2010, after LOFAR is projected to have been operational forapproximately 5 years. Light curves from a combined SKA and LOFAR monitoring programwould be complimentary means of exploring and contrasting IDV and LFV.

LOFAR may also probe novel regimes of scattering. Typical analyses of scattering assumethat the scattering medium extends an infinite distance transverse to the line of sight. At lowfrequencies, as the scattering diameter increases, the scattering diameter may begin toapproach the size of the scattering region. If so, the assumption of an infinite scattering regionis no longer appropriate. Such spatially-limited scattering, if detected, would result in theapparent structure of the source being determined by the scattering region. To date,scattering observations have focused on the Galactic plasma. At low frequencies,intergalactic scattering may become detectable. The dominant source of intergalacticscattering toward distant sources is probably the interstellar media of intervening galaxies,however, at high redshifts or low frequencies effects from dense Ly_ clouds may also becomedetectable.

ReferencesBondi, M., Padrielli, L., Gregorini, L., et al. 1994, A&A, 287, 390Rickett, B. J. 1990, ARAA, 28, 561

5.7 PolarimetryThe magnetic field is a major source of pressure in the ISM and controls the flow of chargedparticles in and out of the Galactic disk. Radio continuum polarization data carry important,and often unique, information about the strength and topology of the large scale Galacticmagnetic field, plasma turbulence and the ionized components through the measurement ofFaraday rotation. As such, these observations complement the line-of-sight measurements ofinterstellar scattering. Moreover, additional observational constraints would help motivatefurther MHD modelling, particularly now that computational power is approaching what isneeded to do the complex simulations. Polarization studies could provide insight in thefollowing areas: (1) Disk/halo emission (Parker instabilities, bubbles); (2) Cosmic ray originand propagation; (3) Particle acceleration via reconnection processes; (4) Origin of theGalactic magnetic field (wound up by differential rotation from a primordial seed field?); (5)The existence of large-scale currents (e.g., those thought to be required by the Galacticcenter filaments); and (6) Distribution and temperature of ionized gas (103–107 K).

A recent additional motivation for the study of the diffuse polarized emission of disk galaxiesis the recognition of its importance for high-redshift galaxies. The microJansky population ofradio sources (at a few GHz) appears to be dominated by starforming galaxies at highredshift. The radio luminosity of these galaxies is generated probably by synchrotronemission from their disks. The sources of particles responsible for this emission seem clear(massive star formation, OB stars, SNe and pulsars). However, it is less clear how themagnetic field was formed and how strong it is. Understanding the field in our Galaxy (i.e., aprototypical z = 0 galaxy) is essential to infer correctly the properties of high-z, young diskgalaxies.

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5.8 Recombination LinesIn the 20-100 MHz range both hydrogen and carbon recombination lines occur from highquantum number transitions (in contrast to the n < 200 transitions observed at centimeterwavelengths). At such high quantum numbers, atoms have radii of order 0.1 mm and areextremely sensitive to their surroundings, making them excellent probes of the ambientphysical conditions. Furthermore, at these low frequencies, most of the observedrecombination lines are due probably to carbon rather than hydrogen. Sufficiently broadbackends will allow simultaneous observations of various line sequences. At the lowestfrequencies, the level populations are determined by collisions with free electrons rather thanby radiative transitions; the lines are thermalized and can be seen in absorption againstbackground sources. The quantum number at which the lines change from being in emissionto absorption depends on the balance of collisional excitation (temperature and density) andradiative excitation (local radiation field).

A number of Galactic regions that produce these lines have been found, including a largeregion that stretches 40° along the Galactic plane in the inner Galaxy. Recombination lineobservations in the direction of individual sources, notably Cas A, have yielded importantinformation on the condition of the diffuse interstellar medium at low densities, ne < 1 cm−3,and temperatures, Te < 100 K. However, all observations of these lines have been made withfilled apertures with extremely low angular resolutions. The complexity of the analysisrequires resolutions better than 1” for further progress. This is needed in particular todiscriminate between two sets of models, one corresponding to a cold ISM phase associatedwith molecular hydrogen, and one corresponding to a slightly warmer ISM phase associatedwith neutral atomic hydrogen. The very central portion of LOFAR will have sufficient surfacebrightness sensitivity for detection of these lines and would provide much needed angularresolution to map their distribution. The sensitivity of the LOFAR will also be sufficient toextend such studies to nearby galaxies.

5.9 Neutron Stars and PulsarsLOFAR provides an excellent opportunity to re-open the study of pulsars at low frequencieswith greatly enhanced sensitivity and modern analysis techniques. Despite the fact thatpulsars were originally discovered at low frequencies (Hewish et al 1968) the vast majority ofsubsequent studies have been carried out at higher frequencies and presently there are fewobservatories which are able to make low frequency observations of pulsars. The move awayfrom low frequencies has been due mainly to the problem of dispersion of the emission from apulsar during its traverse through the interstellar medium. As this effect, which causes pulsesto be smeared out, scales with frequency to the power three, low-frequency observations areparticularly affected. However recent improvements in storage capacity and computationalpower mean that techniques which completely compensate for this dispersion can now beimplemented (Hankins 1974). While pulse broadening due to interstellar scattering will still bean issue at the lowest frequencies, a large fraction of pulsars will be able to be studied withunprecendeted time resolution at low frequencies.

The flux density of pulsars typically increases steeply as we go to lower frequencies beforethey turn over somewhere in the range 100 to 250 MHz (e.g. Malofeev et al. 1994) while thereare even some pulsars for which no such break in the spectrum has yet been seen down tofrequencies as lows 50 MHz. Thus all these sources are either brightest in the LOFAR bandor below. Combined with the excellent sensitivity of LOFAR this means that LOFAR will beable to make some of the most sensitive measurements ever of pulsars. It also indicates thatLOFAR will make an excellent instrument for searching for low-luminosity or extremely steepspectrum sources.

5.9.1 Searching for new pulsarsLOFAR's sensitivity and unique frequency range combined with its sky coverage make it anideal instrument for undertaking pulsar surveys. The only caveat being the limitationsimposed by scattering in the interstellar medium restricting our detection capabilities for themore distant sources.

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The apparent detection of the Geminga pulsar only at the specific frequency of around 100MHz (Malofeev & Malov 1997; Kuzmin & Losovskii 1997), and the existence of pulsars likeB0943+10, which have a flux-density spectrum with a spectral index steeper than -4.0(Deshpande & Radhakrishnan 1994), suggests that there may be quite a number of pulsarswhich are presently only detectable at lower frequencies. This strange behaviour could beeither intrinsic to the emission mechanism of pulsars, or due to other geometrical effects. Themost likely reason may be the latter, where the pulsar emission cone width increases as wego to lower frequencies as seems to be indicated by the the radius-to-frequency mappingseen at higher frequencies and the dipole geometry of the pulsars magnetic field. This effectbecomes very significant at low frequencies where the ``beaming fraction'' of pulsarsincreases considerably. Thus, pulsars which may not be beamed towards us at higherfrequencies may be detectable only at the lower frequencies. Discovering these pulsars willnot only be vital for understanding the geometry of pulsar emission regions but will improveour knowledge on the total number of radio pulsars in the Galaxy.

Even with our present understanding of luminosity model of pulsars we expect that LOFARwill be able to detect a large number of pulsars. With the projected sensitivity of (~55 K/Jy at150 MHz) Monte Carlo simulations of pulsars ``detectable'' by LOFAR indicate that $\sim$1500 could be discovered (above the declination of -25 degrees). The sensitivity of LOFARwill thus enable us to greatly extend our knowledge of the pulsar luminosity function to lowerluminosities. Improved understanding of the luminosity function is vital for determining howmany pulsars there are in the Galaxy, this has important implications for our understanding ofboth the formation mechanism of pulsars but also for other experiments like gravitationalwave detectors.

LOFAR will also open up the possibility to serach for low-frequency emission from moreexotic neutron stars. A sub-class of neutron stars with magnetic field strengths ~1015G whichare currently only known to emit at X-ray and γ-ray wavelengths has recently beendiscovered, these are the so-called magnetars (e.g. Kouveliotou et al. 1998) and anomalousX-ray puslars (e.g. Israel et al. 1999). A recent claim of a detection of one of these sources asa radio pulsar at 100 MHz (Shitov 1999) challenges our ideas about whether the radioemission mechanism of pulsars works at such high magnetic field stengths (e.g. Baring &Harding 2001). Furthermore there are a large number of supernova remnants (SNRs) whichhave no compact counterparts, as we believe that there is a strong link between the formationof neutron stars and supernova remnants, the nature of the sources produced by theseexplosions is thus important. For example do they all become pulsars or is it simply that thebeam does not cross our line of sight. These remnants could potentially contain low-luminosity or steep-spectrum radio pulsars and recent very long integrations have revealedtwo low-luminosity pulsars in SNRs (Camilo et al. 2002a,b). With a sensitive instrument suchas a LOFAR many more such sources may be discovered thereby improving ourunderstanding between the relationship of the supernova remnants and the compact objectsthey form, and the percentage of so formed neutron stars that become pulsars. A similarissue is the nature of the unidentified EGRET gamma-ray sources many of which arepresumed to be neutron stars (e.g. Halpern et al. 2002). A survey of SNRs as well asunidentified EGRET sources and the vast number of γ-ray sources expected to be discoveredwith GLAST (e.g. Baring & Harding 2000) with LOFAR would certainly provide an excellentwindow on the pulsar population.

It may also be possible with LOFAR to detect pulsar's in M31. Assuming a pulsar luminosityfunction similar to that of our own Galaxy (Lyne et al. 1998) and a typical spectral index of -2we find that the brightest pulsars will have a flux ~0.15 mJy at 120 MHz and should bedetectable by a LOFAR with sensitivity as given above, in an observation of 4 hours.

5.9.2 Emission physicsThe instantaneous sensitivity of LOFAR makes it a very attractive instrument for studyingindividual pulses from pulsars. Studies of single pulses on timescales ranging from a fractionof a pulse period (i.e. microseconds) to many pulse periods (up to hours) are vital to ourunderstanding of the pulsar emission mechanism.

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Pulsars show a number of variations in their pulse to pulse characteristics which are believedto be associated with the emission mechanism: nulling is seen in some pulsars andcorresponds to periods when the pulsar stops emitting completely; mode changing occurs insome pulsars and corresponds to a completely distinct emission mode leading to a differentaverage pulse profile from that normally observed; drifiting subpulses, the average pulseprofiles of pulsars are often made up of many subpulses, in a small number of pulsars thesepulses exhibit a regular drift pattern through the average pulse window (Deshpande & Rankin1999). In general LOFAR will enable us to expand the number of pulsars for which we canstudy individual pulses by an order of magnitude. As well as greatly increasing the number ofpulsars and thereby increasing out statistical sample, the low-freqeuncy nature of theobservations are important also.

The most popular model of these subpulses is that they lie on a ring centered on themagnetic axis of the pulsar and rotate around like a “carousel” (Ruderman & Sutherland1974). Deshpande & Rankin (1999), in observations made at at 400 MHz, show that usingextremely high signal-to-noise observations they were able, for the first time, to make a mapof the pulsar emission region or “carousel” of PSR B0943+10. Follow-up work has beencarried out by Asgekar & Deshpande (2001) at the very low-frequency of 35 MHz. At 35 MHzour sightline probes a different sight line and thus provides significant new information aboutthe emission region. Future studies of this phenomenon in this pulsar and others with LOFARwould not only have greater sensitivity but could also be made with simultaneous frequencies,giving us information on the emission map over a larger radius. Present studies have shownthat the drift rate is less and that the drifting behaviour is less stable at low frequencies.However because long observations are required to study these phenomenon and themajority of low-frequency arrays presently working are meridian transit instruments, LOFAR'sability to track pulsars will allow it to make a significant contribution to clarify these issues.

Another area where the raw sensitivity of LOFAR will provide a leap forward is in the study ofmillisecond pulsars. Millisecond pulsars are old neutron stars which have been spun up torapid rotation rates (< 25 ms) by accretion from a binary companion star. This accretionprocess has reduced their magnetic field strengths by 4 orders of magnitude below those ofthe younger pulsars we have been discussing previously. The more rapid rotation rates andthe in general lower luminosity of millisecond pulsars means that virtually no studies of singlepulses has been made of this evolutionarly distinct pulsar popultaion. LOFAR will enable us tostudy single pulses from approximately 20 millisecond pulsars. These observations will bemade at the higher LOFAR frequencies to eliminate the problems of scattering. This willrepresent the first chance to determine if millisecond pulsars exhibit the mode changing,drifting subpulse and nulling characteristics of their slower spinning brethren.

The study integrated pulse profiles, an average of many individual pulses, will also be animportant program for LOFAR. Comparison of the arrival times of multiple low-frequencypulse profiles from a particular pulsar will allow us to determine the dispersion measure, ameasure of the integrated number of electrons along the line of sight, to 1 part in tenthousand. This will allow us to not only study the dispersive effects of the interstellar mediumbut also any affects due to propagation through the pulsar's magnetic field itself.

The integrated pulse profiles also exhibit a phenomenon called radius-to-frequency mappingthat pulses broaden as we go to lower and lower frequencies. This is believed to be anindication that the pulses are being emitted further out in the pulsars magnetosphere wherethe dipole field lines are most divergent. In some pulsars this phenomenon is only observedwhen we get to the lowest frequencies and thus observations with LOFAR, especially atfrequencies below 100 MHz, can provide excellent evidence of the phenomenon. Acomparison with this broadening effect and the frequency where the pulsar's spectrum turnsover will also be a constraint on the emission process.

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Figure 5.1 Average polar cap map of emission of PSR B0943+10 using a 125 pulse sequence and thecartographic transformation procedure described in Deshpande & Rankin 2001. These observationswere made using the Gauribidanur Telescope at 35 MHz. Twenty sub-beams can be discerned in thismap which highlights the power of low-frequency observations in allowing us to greatly improve ourunderstanding of pulsar emission physics and the geometry of pulsar magnetospheres.

5.10 Jupiter and Extrasolar Jupiters

5.10.1 Jupiter MagnetosphereJupiter has powerful radiation belts which produce bright synchrotron radiation. Althoughmeasurements at a variety of radio frequencies have been used to determine the energyspectrum of the radiating electrons, a number of questions about their distribution and sourceremain uncertain. The emission has been extensively mapped at cm wavelengths andreconstructed in 3 dimensions using tomographic techniques (de Pater & Sault 1998; Sault etal. 1997). This synchrotron emission was also recently detected at 74 MHz using the VLA(dePater 1999). Above about 1 GHz, the spectrum shows a standard power law with aspectral index of ~ -0.5 but it flattens and then turns over at lower frequencies. This spectrumhas been empirically fit with a four parameter model (van Allen 1976) but refinement of thefitting parameters and their relation to physical conditions remain to be explored. The mostgenerally accepted causes of the decrease in low energy electrons are pitch angle scattering,radial diffusion processes, and accleration at instabilities in the field and plasma. Theseprocesses can all vary with position in the magnetosphere and the presence of satellites.

It is thus very important to image Jupiter's magnetosphere at several frequencies below 300MHz, where the spectrum is changing, at the same time and with the same spatial frequencycoverage. This is the only way that the different effects can be separated and theirimportance evaluated. The requirement for similar timing is necessary because, as well aspossible long term variations with solar distance, Jovian seasons (small but present), andsolar stimulation (Klein et al. 1998), the inclined and offset magnetic dipole significantlychanges the observed emission with Jupiter's 10-hour rotation period. In addition to helpingunderstand the energetics of Jupiter's magnetosphere, these data may have inplications for

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establishing how the effects of space weather may get transferred through the earth'smagnetosphere.

5.10.2 Decameter BurstsThe source of bursts of radio emission from Jupiter at decameter wavelengths still remains amystery more than 40 years after their discovery. Many observational parameters have beendetermined but we still don't know the actual location of the source nor the detailed emissionmechanism. Until we have the high angular resolution afforded by LOFAR we will not be ableto determine the key positional information.

These bursts of decametric radio emission occur from frequencies below the ionosphericcutoff up to 40 MHz. Their repetition is tied to the spin rate of the planet with modulationcaused by the viewing aspect from earth plus the location of the satellite Io, and to a lesserextent some of the other inner satellites, with respect to the line of sight to earth. Many ofthese data indicate that the radiation may be highly beamed. Any possible solar dependenceis unconfirmed. Early VLBI experiments showed that the source size is small, probably lessthan about 500 km, but they had no phase reference and so the source locations remainunknown (Dulk 1970; Lynch et al. 1972).

Jovian storms have typical bandwidths of a few MHz, drift either up or down in frequency withrates of about 1 MHz/minute, and can last for tens of minutes. Individual pulses, however, canhave bandwidths of only tens of kHz and last for fractions of a second. The brightness peaksat a fequency of about 10 MHz; typical flux densities at 20 MHz would be about 106 Jy (seee.g. the review by Carr & Desch 1976).

The detailed emission mechanism is not known. The radiation is highly circularly polarizedwhich may suggest cyclotron radiation from energetic electrons orbiting Jupiter's magneticfield lines in the ionospheric regions of the planet. They appear to be dumped there throughperturbations by the satellites passing through Jupiter's extensive magnetosphere. Thecoupling between Io's orbital motion and Jupiter's ionosphere, the instability mechanismwhich can amplify the electromagnetic waves, and the propagation of the radiation throughthe plasmasphere are still problems to be resolved.

Clearly, a knowledge of specifically where the source lies plus its variation with rotation andIo's location will greatly aid efforts to characterize the interrelated mechanisms resoponsiblefor these decameter bursts from Jupiter. The good resolution of LOFAR – about 1/10th ofJupiter's disk for a 400 km baseline at 30 MHz - would be very valuable for monitoringchanges in the source position with time. The emission is strong so that individual bursts canbe characterized as well as whole storms. For example, does the position shift as thefrequency drifts, how far are the feet of the active flux tubes from Io's position, etc.?

ReferencesCarr, T. & Desch, M. 1976, in Jupiter, ed. by T. Gehrels, 693Dulk, G. 1970, ApJ, 159, 671Klein, M., Bolton, S., Gulkis, S., & Levin, S. 1998, BAAS, 30, 1079Lynch, M., Carr, T., May, J., Block, W., Robinson, V., & Six, N. 1972, ApLett, 10, 153de Pater, I. 1999, in Perspectives on Radio Astronomy: Science with Large Antenna Arrays,

ed. by M. P. van Haarlem, 327dePater, I. & Sault, R. 1998, JGR Planets, 103, No. E9, 19, 973Sault, R., Oosterloo, T, Dulk, G., & Leblanc, Y. 1997, A & A, 324, 1190Van Allen, J. 1976, in Jupiter, ed. by T. Gehrels, 928

5.10.3 Extrasolar PlanetsThe recent detections of extrasolar planets have stimulated the imagination of manyastronomers and members of the public as well. The existence of places possibly similar tothe earth brings up the question of our uniqueness and all the implications thereof. Theresults to date, however, are surprising in that all the detected planets appear to be manyJovian masses and to lie well within 1 a.u. of their parent stars. It is difficult to understand howthey could have formed in such tight orbits without tidal disruptions or have been perturbed

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into stable orbits that close to a more massive object. It may be that such planets are rare andhave been detected by observational selection but maybe the solar system is the unusualcase. The method of detecting cyclic variations in radial velocity of a star to indicate orbitalmotion, requires a large mass for the planet and a small orbital radius in order to produce adetectable shift in the star's motion. Detection of decameter bursts similar to those fromJupiter might offer a way of detecting lower mass planets at any distance from their stars.

Five solar planets have magnetospheres although only Jupiter produces strong decameterbursts. The intensity of these bursts should depend on the magnetic moment of the planetwhich is likely proportional to the mass and also the rotation rate. Injection from the stellarwind is at least partly responsible for the magnetospheric particle population; this, in turn, isproportional to the inverse square of the distance from the parent star and the stellar activity.Finally, the passage of satellites through the magnetosphere affects the dumping of theparticles into the ionosphere. It is clear that we need to study Jupiter thoroughly with LOFARto find the location of its burst radiation which will help us to more thoroughly assess theemission processes and better choose other candidates for detection of similar planetarysystems.

As a quick assessment of the relative possibilities for detection of bursts from extrasolarplanets, we note that Jupiter has the strongest magnetic field but the other planets also havedifferent geometrical effect: the earth is closer to the sun but does not have any satelliteswithin its magnetosphere; Saturn does have weak hectometric bursts detected by Voyagerbut its magnetic axis is aligned with its rotational axis rather than tilted and it is further fromthe sun; Uranus and Neptune have highly tilted magnetic as well as rotational axes so boththeir stimulating and viewing geometries may be unfavorable.

One good planet in the solar system with decameter bursts readily detectable by LOFAR isvery promising, however, particularly if we wish to begin the search for extrasolar planets likethose detected to date by the radial velocity techniques. They have larger masses thanJupiter (and thus likely larger magnetic fields) and are closer to their parent stars (with thus agreater cross section for the stellar wind). A reasonable detection rate is certainly possible.

It should be noted that the sun produces noise storms of comparable brightness in the samefrequency range but they are much more sporadic and do not have the precise repetition withrotation rate. Thus, it will be necessary to monitor each candidate for a few months to searchfor periodicities appropriate for a planetary rotation, say a few hours to several days.

With typical bursts of about 106 Jy at 20 MHz for a planet at Jupiter's distance, (they can be afactor of 10 higher) we can expect 0.4 mJy at 1 pc or 4 microJy at 10 pc. This may makedetections marginal as we have only about 5 MHz bandwidth and a few minutes per burst.

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6 THE SUN AND SOLAR-TERRESTIALRELATIONSHIPS

6.1 IntroductionThe magnetic activity of the Sun is responsible for the production of Coronal Mass Ejections(CMEs), magnetized clouds of dramatic energies hurled into space, which when hitting theEarth's magnetosphere have severe impacts on both space-borne and ground-basedtechnological systems. The coupling between disturbed solar wind and terrestrialmagnetosphere which causes these magnetic storms on the Earth and which are knowncollectively as "space weather" is electrodynamic in origin. Accurate prediction of the stormsis of large economic importance. A direct way to image CMEs and to determine whether theyare moving on a collision path towards Earth is by detecting the solar light scattered from thedensity enhancements in CMEs, and this principle is worked out to great sophistication inorbiting and planned satellites. So far, it has appeared difficult to predict whether or not aparticular CME is going to hit the Earth. The relatively large number of false alerts makes thepresent space weather forecast of little value. For example, it is difficult to decide fromsatellite data alone whether a "halo" CME is moving toward the Earth or, alternatively, awayfrom both Sun and Earth on the other side of the Sun.

Solar wind, terrestrial magnetosphere and ionosphere are elements of one coupledsystem which causes space weather. Not only is a better understanding of this couplingrequired for an accurate forecast and the mitigation of economic damage the magnetic cycleof the Sun probably also has a direct but ill-understood effect on the terrestrial climate itself.This fundamental problem is receiving increasing attention recently, and presents one of themost urgent challenges in the fundamental study of solar-terrestrial relations. There is nowincreasing evidence for a correlation between variations in the global mean temperature ofthe Earth, solar irradiance in the UV, smoothed sunspot numbers, the open magnetic flux inthe solar wind, the fast solar wind, and auroral activity over a considerable period. Also thereis an anticorrelation with modulations in CR flux caused by the solar magnetic field. Which ofthese factors is instrumental in causing the climate changes is as yet unknown. One of thelinks is the ionizing potential of the CRs as they are modulated by the magnetic field in theheliosphere and their effectiveness in creating aerosols onto which water droplets form.Another possible link are the changes in electric current system brought about by interactionof CMEs that have a southward directed component of the interplanetary magnetic field whichleads to magnetic reconnection between CME and terrestrial magnetosphere on the day-sideand the onset of so-called magnetic substorms. How and if these changing currents influencethe climate, eg. by setting up convection winds, remains unknown at present.

LOFAR will contribute in these areas in a number of ways:1. by accurate predictions of Earthward-bound CMEs and estimating their arrival times from

direct imaging of the free-free and synchrotron emission in the CMEs emitted at radiowavelengths directly. It should be noted that even at a low observing frequency of 10MHz the corresponding plasma frequency in the solar wind is only at a distance of a fewsolar radii from the Sun, a mere 5% of the distance from Sun to Earth, and therefore,accurate determination of the future path of the CME still leaves at least a day formeasures to be taken. Also, radio detection of a CME implies that the disturbance is onour side of the Sun and does not suffer from the ambiguity of satellite detection;

2. by detailed tomography of CMEs and related disturbances using imaging and scatteringtechniques which brings within reach realistic MagnetoHydroDynamic modelling of thedisturbance with the terrestrial magnetosphere. Such fundamental research is necessaryto understand the evolution of geomagnetic (sub)storms, and more general, if and howthe solar wind magnetic field influences the terrestrial climate, either directly through theelectric circuit, or indirectly via CR modulation;

3. by active radar detection of CMEs - as distinguished from "passive" detection of a CMEby its own emission - as proposed in the active radar experiment LOIS (2002). Thedetections of temporal and frequency characteristics of radar echos from the solar wind atlow frequencies (in view of the small densities) are expected to provide a sensitivediagnostic of the solar wind and CMEs similar to the current use of radar for meteoritesand asteroids which allows their detection and accurate determination of their orbits and

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shapes. Also, active radar will permit unravelling of the plasma turbulence in the solarwind by a variety of non-linear wave coupling processes, and reveal the microscopicnature of effective resistivity and viscosity.

6.2 Space WeatherSpace weather is the name given to the collective of magnetic storms and disturbances at theEarth. It is governed by the strongly varying solar output that hits the Earth and its environs.Actually, the magnetized solar wind, the terrestrial magnetosphere and the ionosphere areelements of one electric circuit, and variations in one element is of direct consequence on thecurrent in the circuit. As a result global magnetic disturbances reach the Earth's surface,electric currents are induced in conductors on the ground, and energetic particles areproduced (Daglis et al. 2001; Daglis 2001). Space weather hazards include malfunction anddamage of telecommunication including long-distance telephone cables, navigation,surveillance satellites (spacecraft charging, single-event upsets), high-voltage power gridsand danger to astronauts and passengers of airplanes at high latitude and altitude. They alsocan affect over-the-horizon radars, HF, VHF, and UHF communications, the use of GPSsatellites, increased satellite drag by atmospheric heating and corresponding inaccuracies intracking orbiting objects, and deterioration of magnetic-torque attitude control systems ofsatellites. The main causes of space weather are strong variations in UV radiation thatchange the altitude of the ionosphere, and disturbances in the solar wind such as CoronalMass Ejections (CMEs). These solar products are largely related to the magnetic activity ofthe Sun which has a cycle of 2*11 years. Another fundamental aspect of the solar-terrestrialrelationship is a possible connection with the Earth's weather and tropospheric climate. Notonly does the magnetic activity on the Sun have a strong impact on terrestrial conditions, alsoit presents the nearest laboratory for magnetic explosions (reconnection, particle acceleration,Joule heating and electric motors) similar to processes in magnetospheres of such distantobjects as compact stars and accretion disks in X-ray binaries and AGN (Kuijpers; Pavlidou etal. 2001; Kuijpers 2001).

6.2.1 LOFAR and Coronal Mass EjectionsCoronal Mass Ejections (CMEs) are the most energetic explosions on the Sun. On a timescale of minutes a huge magnetized plasmoid of dimensions of 106 km and total mass oforder a few times 1012 kg is propelled outwards at a speed of 500 km/s or more, representinga total energy of 1024 J. The nature of the explosion is believed to be MagnetoHydroDynamicin origin, and is the result either of an MHD instability or a loss of equilibrium which ariseswhen the free energy in the CME becomes too large with respect to the line-tying effects ofthe magnetic field at the phostospheric boundary. Apparently, magnetic reconnection causesthe magnetic structure to relax to a lower energy state thereby converting part of the freemagnetic energy into motion of the plasmoid, and another part into particle acceleration whichcan be observed as gyrosynchrotron emission from the moving CME.

As the CME is a strong density enhancement in the first place it distinguishes itself byan excess of thermal bremsstrahlung modified by gyroresonance effects, which under typicalconditions is optically thin above an observing frequency of 50 MHz. As a result,simultaneous observations at various frequencies (imaging spectroscopy) allows tomographicimaging of its spatial and temporal structure (Bastian & Gary 1997). This would complementthe images from the proposed Frequency-Agile Solar Radiotelescope (FASR) which willoperate between 0.3 - 30 GHz (Hurford et al. 1999) and would allow a detailed solution of theactual formation process in terms of magnetic reconnection and its consequences. Also, suchobservations are important for realistic 3D MHD modelling of the interaction of a CME and theEarth's magnetosphere (Keppens & Goedbloed 2000; Poedts 2001), and its effect on themagnetospheric ring current and the development of magnetic storms. To sketch thetechnical difficulty of a realistic MHD model: the conductivity of some of the elements canexhibit five orders of magnitude variations. Another important diagnostic of CMEs withLOFAR will be scattering measurements against cosmic background sources - angularbroadening and scintillation measurements - to determine its internal turbulence.

6.2.2 The Solar Magnetic Cycle and the Terrestrial ClimateThe magnetic activity of the Sun as measured by sunspot numbers over the period 1880 -1993 correlates well with global mean temperature variations at the Earth (Soon et al. 1996).

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The magnetic activity also correlates well with solar irradiance variations in the EUV asmeasured from 1978 onwards (Solanki & Fligge 2000) which in their turn correlate well withthe amount of open magnetic flux and anticorrelates with the CR modulation at the Earth bythe solar wind magnetic field (Lockwood 2002). The flux of CRs correlates with the globalaverage of low cloud cover over cycle 1983 - 1994 (Marsh &Svensmark 2000). The openmagnetic flux comes mainly from coronal holes which are the sources of the fast solar wind(Wilhelm et al. 2000). Finally, the smoothed sunspot numbers correlate well with visualobservations of auroral activity over the past 1000 years (Pulkkinen et al. 2001; Pulkkinen2001). At present, it is not clear which of these quantities is causing the climate variations: theUV irradiation directly (Soon et al. 1996), the time variation of the ionising potential of CRswhich modulate the number of cloud condensation nuclei and aerosols (Marsh & Svensmark2000; see also the "CLOUD" proposal by Fastrup et al. 2000), or the magnetic activity viadisturbances such as CMEs in the solar wind. Because of its superior potential to image thephysical consditions in CMEs (see previous section) LOFAR will be in an excellent position tocontribute to the investigation of the possible relation between solar wind disturbances andclimate.

6.2.3 LOFAR and CME warningWhen a fast CME happens to impact on the Earth's magnetosphere and when its prevailingmagnetic field is directed southward, i.e. opposite to the terrestrial field in the magnetosphere,over a prolonged period of time a geomagnetic storm is created whereby acceleratedparticles are dumped into the magnetosphere and large inductive voltages set up across thefield lines. The physical process underlying this development is magnetic reconnection of fluxtubes at the day-side. These effects can be hazardous to humans and satellites in space aswell as to power lines on the ground. Naturally, strong pressure has built up in the past fewyears to establish a timely warning system. So far, CMEs have been detected most easilywith coronographs aboard satellites such as SOHO as their density enhancements scattersolar light in all directions. To determine whether or not a particular CME, usually of the halotype, was going to hit the Earth has proved not to be a simple matter. One problem has beenthat it is difficult to find out if the CME is moving toward the Earth, or rather moving away bothfrom Sun and Earth. Finding out the "geo-effectiveness" of a CME is one of the major issuesin space weather.

At the end of 2002, NASA will launch SMEI (Webb et al. 2002) to accurately predictthe arrival of CMEs at Earth. This satellite does not use a coronograph but images scatteredsunlight and uses a tomographic "time-dependent" analysis to detect ionized clouds and theirmotion towards Earth down to 1011 kg (Jackson & Hick 2000). The cameras have a hugedynamic range to reduce stray-light to less than one part in 1015 that can distinguish a CME orsolar wind perturbation at large distances from the Sun. In the near future STEREO will belaunched which consists of two satellites and will produce stereoscopic images of the solarwind, and of CMEs as they propagate. Undoubtedly, the predictive power of satellites forCMEs hitting the Earth will improve considerably by these missions. Still, a satellite warningsystem will remain a costly affair, and it is here that LOFAR is expected to contributesignificantly. Note that much of the radio emission from disturbances in the solar corona comefrom near the plasma frequency. Therefore, an observing frequency as low as 10 MHzcorresponds to an electron density of 1010 m-3 or a distance from the Sun of only a few solarradii, still far away from Earth and allowing a warning time of at least a day. At radiowavelengths, it will be immediately obvious if a CME is between Earth and Sun rather thanbehind it. Secondly, the most massive and energetic CMEs are associated with so-calledType II radio outbursts from the shock wave in front of the CME (Cane et al. 1987). TheseType II bursts are a specific form of coherent emission at the fundamental and secondharmonic of the plasmafrequency and can be easily recognized on a dynamic radiospectrum.The behaviour of the images of the Type II radio emission as a function of time and frequencyenable to determ the path of the CME accurately. Further, tomographic techniques will permitthe modelling of the temporal and spatial evolution of a CME in detail from simultaneousmulti-frequency LOFAR measurements of the radio emission, generated inside the CME.Sometimes synchrotron emission - so-called Type IV emission - from accelerated trappedparticles is observed (Bastian et al. 2001) which can be distinguished from the internalthermal bremsstrahlung.

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6.2.4 LOFAR, Active Radar and Space WeatherRadio sounding has now become a sophisticated technique to determine electron densitystructures and their dynamics, primarily in the ionosphere (Hunsucker 1991; Kohl et al. 1996)and recently also in the magnetosphere (Green et al. 2000). The use of active radar atfrequencies above 10 MHz for a timely detection of CMEs directed towards Earth severaldays in advance of possible geomagnetic storms has been advocated by Rodriguez (2000)and a detailed proposal can be found in the proposal for LOIS (LOFAR Outrigger inScandinavia LOIS, Thidé 2002). The corresponding plasmafrequencies still correspond todistances below a few solar radii away from the Sun. The radar technique allows for an earlydetection and is a potentially sophisticated diagnostic of the spatio-temporal structure of aCME or any other moving density disturbance in the solar wind. The idea is to radiate a pulsewhich is narrow both in time and frequency extent and to unravel the complex dynamic radiospectrum that is returned by the propagating disturbance. The return pulse from the Earth-ward propagating structure is expected to be boosted both in ampltude and in inverseduration as compared to the echo from the surrounding corona. Recent experiments have asyet not been conclusive. In principle, the technique is a powerful one, and sophisticated raytracing computations have now been started for the solar wind to demonstrate its feasibility,using the Swedish Institute of Space physics (IRF) ray tracing system "RaTS" (Västberg1997) .

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NetherlandsDaglis, I.A., Baker, D.N., Galperin, Y., Kappenman, J.G., Lanzerotti, L.J.: 2001 EOS 82, 585Fastrup, B. et al.: 2000 CLOUD Proposal, CERN/SPSC 2000-021; SPSC/P317Green, J.L., Benson, R.F., Fung, S.F., Taylor, W.W.L., Boardsen, S.A., Reinsich, B.W.: 2000,in Radio

Astronomy at Long Wavelengths, eds. R.G. Stone, K.W. Weiler, M.L. Goldstein, J.-L.Bougeret, Geophysical Monograph 119, American Geophysical Union, Washington,DC, p. 359

Hurford, G.J., Gary, D.E., Bastian, T.S., White, S.M.: 1999 BAAS 31, 956Jackson, B.V. & Hick, P.: 2000 Adv. in Space Res. 25, 1875Keppens, R. & Goedbloed, J.P.: 2000 ApJ 530, 1036Kuijpers, J.: 1995 Flares in Accretion Disks, in Lect. Notes in Physics 444, 135, Springer-Verlag, BerlinKuijpers, J.: 2001 PASA 18, 407Lockwood, M.: 2002 A&A 382, 678Pavlidou, V., Kuijpers, J., Vlahos, L., Isliker, H.: 2001 A&A 372, 326Poedts, S.: 2001, in SOLSPA 2001 Euroconference: Solar Cycle and Space Weather, ESASP-477, 77Pulkkinen, T.I.: 2001 AGU #SH 52A-07Pulkkinen, T.L., Nevanlinna, H., Pulkkinen, P.J., Lockwood, M.: 2001 Space Sci. Rev. 95, 625Rodriguez, P.: 2000, in Radio Astronomy at Long Wavelengths, eds. R.G. Stone, K.W.Weiler, M.L.

Goldstein, J.-L. Bougeret, Geophysical Monograph 119, American GeophysicalUnion, Washington, DC, p. 155

Solanki, S.K., Fligge, M.: 2000 Space Sci. Rev. 94, 127Soon, W.H., Posmentier, E.S., Baliunas, S.L.: 1996 ApJ 472, 891Thidé, B.: 2002 Radio Studies of Solar-Terrestrial relationships, LOIS Science TeamVästberg, A.: 1997 Investigatioons of the Iospheric HF Channel by Ray Tracing, PhD Thesis,Uppsala

University, Uppsala, Sweden, Apr. 1997, IRF Sci. Rep. 241Webb, D.F., Johnston, J.C., Radick, R.R. et al.: 2002 EOS 83, 33Wilhelm, K., Dammasch, L.E., Marsch, E., Hassler, D.M.: 2000 A&A 353, 749

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