Potential of a Low Frequency Array (LOFAR) for Ionospheric and Solar Observations

ABSTRACT: The Low Frequency Array (LOFAR) is a proposed large radio telescope array that will operate in the 10-240 MHz frequency range. Its 13,000 dipole antennas will be clustered in roughly 100 stations spread over a region 400 km across. Various potential sites for LOFAR in the southwestern U.S., western Australia and the Netherlands are currently being evaluated. Connected through fiber optical links, this interferometric array will be capable of digitally forming multiple simultaneous beams that can be electronically steered, will possess extreme frequency agility, and will provide a flexible system for distributed control, signal processing, monitoring, and remote operation. The array design is driven by radio astronomy goals to achieve a collecting area of one square kilometer at 15 MHz and arc-second angular resolution. At the low frequencies of operation, an adaptive phase-based calibration of the instrument will be required to compensate for ionospheric delays with extremely high precision, using astronomical radio sources observed along many simultaneous lines-of-sight to the stations. These observations will enable the application of radio tomographic techniques to construct a 3-dimensional view of ionospheric structure on fine spatial (~100 meters) and time scales (seconds). Opportunities will also exist for the potential detection and tracking of coronal mass ejections through interplanetary scintillations, and for passive radar observations using FM radio stations as transmitters of opportunity.

J. E. Salah, F. D. Lind, C. J. Lonsdale, MIT Haystack Observatory G. S. Bust, T. L. Gaussiran II, Applied Research Laboratories, the University of Texas at Austin,

M. P. van Haarlem, Netherlands Foundation for Research in Astronomy, K. W. Weiler, Naval Research Laboratory

What is LOFAR?

THE WORLD’S FIRST MAJOR DIGITAL RADIO TELESCOPE

• Frequency coverage: 10 - 240 MHz

• Array of ~ 100 stations spread over 400 km

• Multiple independent beams and observing programs

• Multi-disciplinary science:

• Astronomy

• Ionospheric and Solar-Terrestrial Physics

• Link to high-bandwidth Internet, remote control and operations

LOFAR Project Plans

• Partners: •ASTRON (Netherlands)•NRL•MIT/Haystack Observatory

• Estimated costs ~ $50M – $75M (~66% ASTRON)•Phase 1: Design 2001-2003 (CDR Jan 2004). •Phase 2: Construction 2004-2006 (IOC Jan 2006)

• Site Evaluation: Texas, New Mexico, Holland, Australia

400 km diameter

Remote operations centers

One LOFAR station, ~150 meters100 to 150 dipoles

LOFAR: Overall Layout

Collecting Area =1 km2 @15 MHz

Astronomy:

• High Redshift Universe

• Epoch of Re-ionization

• Bursting and Transient Universe

Ionosphere and Solar-Terrestrial Physics:

• Ionospheric density variations and fine-scale structures

• Detect solar bursts and image CME emission

Some Key LOFAR Science Drivers

Instrument type: Aperture synthesis interferometric arrayLow frequency range: 10-90 MHz, single dual-polarization active dipolesHigh frequency range: 110-240 MHz, analog-phased 16-dipole arrayTotal number of receptors: 13365 in each frequency rangeNumber of receptors per station: Between 81 and ~ 220 per frequency rangeNumber of interferometric stations: Nominally 100 (up to 165).Number of digital beams at each station: Nominally 8, with phased implementationInstantaneous sky coverage at full resolution: 1 steradian at 20 MHz using 8 beamsDigitized Bandwidth: 32 MHz (64 Msamples/sec per receptor)Baseline range: Between 100 m and ~ 400 kmSpectral resolution: 1 kHz

LOFAR Specifications

LOFAR: Conceptual signal path

(Kassim et al., 1993)

Ionospheric wave effect on VLA(20 km baseline) interferometer phase at

74 MHz

0.1% TEC1 rad

Time (minutes)

Ionospheric Waves detected at the VLA

(R. Perley - NRAO, private comm.)

~ 1

E-region irregularities during storm: 22 Oct 99

Manastash Ridge Radar (Lind et al., 1999)

Clark Lake detection of a CME16 February 1986 - 73.8 MHz

(Gopalswamy and Kundu, 1992)

CME Mass Estimate:2.71012 kg

http://casswww.ucsd.edu/solar/forecast/index.html)

These Carrington maps (with heliographic longitude and Carrington rotation number on the horizontal, and latitude on the vertical axis) show solar wind velocity and density at the distance of the Earth from the Sun on 2001/06/05 16 UT . The values to the left of the dashed line are those forecast to arrive at Earth at the dates indicated above the map. The velocity and density at the Earth are shown as traces at the bottom of the display. The maps are derived from a corotating model of the solar wind that is fit to interplanetary scintillation (IPS) velocities and g-levels received daily from STELab, Japan. The display is updated hourly.

(ref: B. Jackson, M. Kojima et al.)

CME Tracking using Interplanetary Scintillations (IPS)

LOFAR will provide:• Large Source Count (thousands)• Multiple Frequencies (distance from sun)• Resolution (~ 100 stations)