association of lunar & planetary observers …...in england. huggins developed the tech-nique of...

Association of Lunar & Planetary Observers Association of Lunar & Planetary Observers



































Guidelines for theObservation ofMonochromaticSolar Phenomena

A Handbook of the Association

of Lunar & Planetary Observers

Solar Section.

January 2010

3rd Edition

Edited by

Jamey JenkinsAsst. Coordinator, ALPO Solar Section

Originally compiled by

Randy Tatum

Established 1947

Guidelines for the Observationof Monochromatic Solar Phenomena

i© 2010 Association of Lunar & Planetary Observers - All rights r eserved.

Edited by

Jamey JenkinsAsst. Coordinator, ALPO Solar Section

Originally compiled by

Randy Tatum

Established 1947

A Handbook of the Association of Lunar &Planetary Observers Solar Section.

January 2010

Acknowledgements

ii © 2010 Association of Lunar & Planetary Observers - All rights r eserved.

Welcome to the world of solar astr onomy. The purpose of this book is to bridge the gap betw een the casual andserious observer wishing to contribute to the kno wledge of the nearest star, our Sun. We are a division of the Association ofLunar and Planetary Observers, organized by Walter Hass in 1947; this Section being established in 1982. The function ofthe Solar Section is to stimulate, organize, and disseminate amateur work in the field of solar morphology. Through thearchiving of solar observations we provide a resource for the professional community to supplement their r esearch programs.While we do not offer recommendations regarding sunspot counting or radio flar e patrolling, we do accept and archive sub-mitted observations of that nature from observers. Any member wishing to inv olve themselves deeply in such work shouldadditionally contact the American Association of Variable Star Observers (AAVSO) at 49 Bay State Road, Cambridge, MA.02138 for guidance. Many of our observers participate in both organizations.

Solar morphology is a par ticularly rewarding field of study for the amateur astr onomer since the features of the Sunare the most active and changing in the whole of the solar system. B ecause of this dynamic, solar activity r equires diligentobserving. Some work can be done within the space of a day or two while other pr ojects require a commitment of manydays, often consecutive. Neither type of observing is any more important than the other, so observers that make a contribu-tion either way are encouraged to do so. The work of the Solar Section and consequently the focus of our effor ts is therecording of visual and photographic obser vations of the Sun. There is a particular emphasis on photographic observationsin white and monochromatic light since these are of the most use to the pr ofessional community. Space limitations willrequire some presumptions on our part that you, as an observer are familiar with astronomical terminology and principles.If you are a novice please contact the Solar Section Coordinator for guidance.

The preparation of this booklet required advice from a number of professional and advanced amateur astronomersto insure that the work of the Solar Section would have immediate and lasting value to astronomy. We gratefully acknowl-edge the support and aid of those listed belo w. For our observations to retain value, it will take dedication and commit-ment from our observers towards producing reliable data that will, by virtue of its own high quality be in demand no w andin the future.

—ALPO Solar Section

Contributors

Richard Hill Jeffery Sandel Kim HayGordon Garcia Jen Winter Art WhippleRick Gossett Jamey Jenkins Monty LeventhalEric Roel Fred Veio Vincent ChanChristian Viladrich Randy Tatum Howard EskildsenPhil Rousselle Ginger Mayfield Ralf Vandebergh

Dr. David Hathaway, NASA/Marshal Space Flight CenterBig Bear Solar Observatory

Larry Combs, SESC

iii© 2010 Association of Lunar & Planetary Observers - All rights r eserved.

Table of Contents

Acknowledgements ..........................................................................................ii

Contents ..........................................................................................................iii

Monochromatic Observing...............................................................................1

Chromospheric Features..................................................................................5

Observing Hints..............................................................................................10

Suggested Reading........................................................................................13

Monochromatic Observing

Monochromatic light is wave-length specific light. Whereas white lightobservervations are done with a wide sam-pling of color from the solar spectrum, themonochromatic observer is using only acomparatively thin slice of light from thewhole of the spectrum.

Since 1982, the A.L.P.O. SolarSection has been collecting and r educingsolar observations to provide useful data toprofessional and nonprofessionalresearchers. The original intention of theSection was to collect white light data ofactive regions. Since that time many ofour observers have become activelyinvolved in more advanced methods ofsolar study; such as spectroscopic work andimaging in the light of Hydrogen alpha(6562.8Å) or the Calcium K-line(3933.7Å).

The Sun is a very beautiful andcomplex object to study and the v ariety ofdifferent phenomena to observe in itsatmosphere are simply breathtaking attimes. Magnetic fields play an importantrole in the understanding of solar activity.In the convection region, below the photo-sphere, gas motions dominate magneticfields, but the opposite is tr ue in the solaratmosphere above the photosphere.

On the practical side, monitoringsolar activity is important to our under-standing of solar/terrestrial relationships.In truth we live within the atmosphere ofthe Sun. The solar wind, an ionized gasthat flows from coronal holes, carries theSun's magnetic field through the solar sys-tem. The connection between activity onthe Sun and the Earth's magnetic field iswell established. Coronal holes, disappear-ing filaments (eruptive prominences) andsolar flares cause geomagnetic storms andaurora. Major flares cause communicationblack outs, power surges and threatenspace missions. The effects of activity onthe weather and life in general has y et to

be proven but the strong connection is sus-pected. Historical records show long peri-ods when the 11-year sunspot cycle is sup-pressed, or absent. These periods may betied to ice ages on Ear th. The Sun is theonly celestial body in which we physicallyinteract (except for meteors and the occa-sional asteroid or comet).

The monochromatic information is divided into three sec-

tions. The first section is an overview ofthe history of monochromatic observationwith emphasis on instrumentation. Thesecond section describes the numeroussolar features to be observed. Several hintsand suggestions pertaining to monochro-matic observing are discussed in sectionthree.

As always caution must be exer-cised when observing the Sun. The dan-gers present with white light observationare still a hazard with monochromaticobserving.

History and Instrumentation

Prior to the year 1868 solarastronomers could observe prominencesonly during the few minutes of totality ofthe eclipsed Sun. Knowledge concerningthese features hence progressed slowly.The question of whether the prominenceswere caused by the Sun, Moon, or an opti-cal illusion was solved at the eclipse of1860 by photography. Early eclipseobservers had speculated that prominenceswere lunar clouds, or mountains on theSun! During the eclipse of 1868, in I ndia,French astronomer Janssen observed thebright emission lines of a prominence atthe Sun's limb, spectroscopically. He wasso impressed by the brightness of theprominence in the Hydrogen alpha linethat he tried to observe the emission inbroad daylight after the eclipse. H e foundthat by placing the spectroscope slit acrossa prominence, he could trace out its shape.Janssen shared this discovery with Lockyer


1© 2010 Association of Lunar & Planetary Observers - All rights r eserved.

ToCorona

Chromosphere

ToInterior

Transi tion Zone

Photosphere

2600 km

2300 km

500 km

0 km

The thickness of the v arious com-ponents of the solar atmospher eare illustrated with this diagram.The base of the photosphere is at astarting point of 0 kilometers, thechromosphere begins at a point500 km above that.

presented here

in England. Huggins developed the tech-nique of widening the spectroscope slit,thereby viewing the entire prominenceinstead of a thin slice. There is a limit towhich the slit can be widened, dependingon the spectroscope's dispersion and onthe clarity of the atmosphere. The widerthe slit, the less contrast a pr ominence has.The early prominence spectroscopes wereshort focus and used a train of prisms fordispersion. The slit had to be positionedtangent to the Sun's limb and had to berepositioned several times to observe theentire circumference of the Sun. For over25 years observers like C. Young inAmerica and Fr. Secchi in Rome observedthe Sun with these instruments.

In order to photograph the entirelimb of the Sun, in monochromatic light,George E. Hale designed the spectrohelio-graph in 1891. He used the broad "K-line" of Calcium in the deep blue par t ofthe solar spectrum. The chromosphereand prominences emit light in several spec-tral lines in addition to Hydrogen alpha,such as the H and K lines of Calcium, theyellow Sodium lines, the yellow HeliumD3 line, and the rest of the Balmer seriesof hydrogen lines. The color of promi-nences during totality is intense pink sinceit is a combination of all these emissionlines. The H-alpha line in the orange/r edpart of the spectrum is the dominant emis-sion line. The spectroheliograph creates animage of the Sun on a photographic detec-tor slowly, slit width by slit width, isolatinga view in a single spectral line. The firstslit of the spectrograph makes the darkabsorption lines; and the second slit iso-lates one particular line for study, lettingits light through to the eye or photograph-ic camera. Hale discovered bright cloudsof Calcium on the Sun's disk aroundsunspots. In 1908 he photographed thesolar disk in H-alpha light and disco veredstrong magnetic fields confining the gasaround sunspots.

The optical counterpart of the

spectroheliograph, the spectrohelioscope,was developed about 1924. In this instru-ment, the slits move rapidly and repeatedlyacross the solar image; due to persistenceof vision the eye sees a continuous imagein monochromatic light. Using thisinstrument it is possible to measure veloci-ties of features along the line of sightquickly with a device called a line shifter .For visual work, the best line in which toobserve is H-alpha. The eye is most sensi-tive in the green to red end of the spec-trum, and the other lines ar e too narrow inwidth to be easily used. The importanceof the spectrohelioscope in the field ofsolar/terrestrial relations became evidentwhen radio noise was found to be associat-ed with solar flares in 1942. Ever sinceCarrington and Hodgson had observed thefirst white light flare in 1859, astronomershad suspected a connection between flaresand geomagnetic disturbances. Hale real-ized the importance of monitoring solaractivity and wanted these instruments tobe distributed to observers around theworld.

There are many designs for spec-trohelioscopes. The solar observer shouldselect the design that best suits his needs.Generally, the spectroscope is fixed in ahorizontal or vertical position. Sunlight isfed to it from a single flat heliostat or fr oma two mirror coelostat system. It would bebest that the spectroscope is used in anobservatory to protect it from wind andscattered light. Diffraction gratings with1,200 lines per millimeter will show thou-sands of Fraunhofer lines in the spectrumand are relatively inexpensive. The lensesused may be single element.

In a large sense, the spectrohelio-scope is analogous to your radio or TVreceiver, except that it operates in the visi-ble part of the electromagnetic spectrum.Various chemical elements transmit at dis-crete frequencies, just as do radio and TVstations. The ability of your instrument to"tune in" on specific wavelengths is impor-


2 © 2010 Association of Lunar & Planetary Observers - All rights r eserved.

A typical view of the Sun in thelight of Calcium on 19 July 2004

by Christian Viladrich.

tant to reduce noise in your receiver, or toimprove contrast in your spectrohelio-scope. The advantage of the spectrohelio-scope is that it doesn't limit the obser ver toone spectral line and offers the entir e spec-trum at a reasonable price.

Around 1930, B. Lyot developed anew device to study the Sun, the mono-chromator. These optical filters were madeof quartz and calcite crystals, and had tobe kept at a constant temperature to pro-vide the best contrast view. The early fil-ters had passbands of several angstromsand could only be used to study pr omi-nences. Great skill was required in theconstruction of these early filters and onlya few names in the amateur ranks come tomind as having accepted the challenge ofbuilding them; names such as H enry Pauland Walter Semerau. The H-alpha line isonly .6Å in width. A filter with a pass-band of one angstrom will show promi-nences easily, but only bright flares on thedisc. A .7Å or .8Å filter will giv e gooddisk contrast with bright prominences. A.5Å or .6Å filter gives very good disk con-trast with subdued prominences. Filterswith passbands from 3 to 10 angstroms areadequate for observing prominences andan occulting disc will improve the view.

The development of the interfer-ence filter improved the efficiency of opti-cal filters by reducing passbands to .1Å.Space age production techniques reducedthe cost of these filters and put them intothe hands of amateur astronomers.Modern commercial units (of which sever-al are available at the time of this writing)are based on the principles of light inter-ference.

Interference filters are multilayerthin-film devices. They can be designed tofunction as an edge filter or bandpass filter .In either case, wavelength selection isbased on the property of destructive lightinterference. This is the same principleunderlying the operation of a Fabry-Perotinterferometer. Incident light is passed

through two coated reflecting surfaces.The distance between the reflective coat-ings determines which wavelengthsdestructively interfere and which wave-lengths are in phase and will ultimatelypass through the coatings. I f the reflectedbeams are in phase, the light is passedthrough two reflective surfaces. If, on theother hand, the multiple reflections are notin phase, destructive interference reducesthe transmission of these wavelengthsthough the device to near z ero. This prin-ciple strongly attenuates the transmittedintensity of light at wavelengths that arehigher or lower than the wavelength ofinterest.

In an interference filter, the gapbetween the reflecting surfaces is a thinfilm of dielectric material called a spacer.It has a thickness of one-half wav e at thedesired peak transmission wavelength. Oneither side of this gap ar e the two reflectinglayers. The reflecting layers actually con-sist of several film layers, each of which is a

quarterwave thick. This sandwich of quar-terwave layers is made up of an alternatingpattern of high and low index material,usually zinc sulfide and cryolite, respective-ly. Together, the quarterwave coatingsforming the reflective layer is called astack. The combination of two stacks andthe spacer comprise a one cavity bandpassfilter. The number of layers in the stack isadjusted to tailor the width of the band-pass.

In practice, a single cavity band-pass filter does not exhibit a sharp transi-tion between the passband and out-of-passband wavelengths. To sharpen this



Typical Interference Filter

Glass Substrate

Glass SubstrateProtective Metal Ring

Multi-cavity Bandpass FilterMulti-layer Blocking Filter

Optical Epoxy

cutoff, it is common practice that sev eralcavities are layered sequentially into a mul-ticavity filter design. A multicavity designalso dramatically reduces the transmissionof out-of-band wavelengths. To completethe interference filter, another set of thin-film coatings are applied to the secondsubstrate to block the transmission ofwavelengths that are further away from thepassband of interest. This blocking layer isessential in filters to prevent "shootthrough" of undesired wavelengths fromthe illumination source to the detector.The blocking layers and passband layersare held together in a protective metal caseusing optical epoxy.

Interference filters are sensitive totemperature and tilt. Tilting the filtershifts the passband to the blue wing of theH-alpha line. When one observes in thecore of the line, one is obser ving high inthe chromosphere and sees maximum con-trast of the prominences, filaments, plagesand flares. When a filter is tuned off bandto the red or blue wing, one sees lo werinto the chromosphere, and less H-alphadetail is visible. Doppler shifted featuressuch as surges and active filaments willappear dark in the wings of the line. Thefarther an H-alpha feature is observedfrom the H-alpha core, the greater its radi-al velocity in the line of sight. The twist-ed, violent motions of active prominencesbecome evident when viewed spectroscopi-cally. Often one part is blue shifted andanother is red-shifted, indicating helicalmotions. Sunspot umbrae are well seen inthe core, but small umbrae are sometimeslost in the detail of the chr omosphere.Penumbrae are visible, but the contrast islow. With a filter tuned about 1Å outsidethe core the Sun generally appears as itdoes in white light.

Because interference filters are sen-sitive to tilt, the light passing thr oughthem must be parallel or nearly so . If thelight is not parallel a "sweet spot" of on-band filtering occurs surrounded by

increasingly off-band transmittance. Thisis combated in several designs by operatingat focal ratios of f/30 or gr eater in whichthe light is a nearly parallel beam orthrough the use of a collimating lens sys-tem between the objective and filter. Aprefilter may also be required for protec-tion from ultraviolet light and to provide a"cooler" beam to the inter ference filter.Narrow passband filters are often enclosedin a heated oven for precision tuningthrough temperature control.

Filter Terminology

Bandpass—The range (or band) of wave-lengths passed by a wavelength-selectiveoptic.

Bandpass Interference Filter—An interfer-ence filter designed to transmit a specificband of wavelengths.

Blocking—The degree of light attenuationat wavelengths outside the passband of thefilter.

Center Wavelength (CWL)—The wave-length at the midpoint of the half po werbandwidth.

Energy Rejection Filter (ERF)—A filter thatis placed over the telescope aperture for thepurpose of reducing the heat load andabsorbing UV light.

Etalon—Essentially an optical filter thatoperates by multiple-beam interference oflight reflected and transmitted by a pair ofparallel flat reflecting plates.

Filter Cavity—An optical "sandwich" oftwo partially reflective substrate layers sep-arated by an evaporated coating whichforms the dielectric spacer layer.Interference filters can be constructed withone or several cavities arranged in series.



Full-width Half-Maximum (FWHM)—Thewidth of the bandpass, in nanometers, atone-half of the maximum transmission.

Interference Filter—An optical filter con-sisting of multiple layers of evaporatedcoatings on a substrate, whose spectralproperties are the result of wavelengthinterference rather than absorption.

Oven—An electrically controlled device forregulating the operating temperature of afilter.

Peak Transmission—The maximum per-centage transmission within the passband.

Telecentric Lens—A supplementary lensintended to create parallel light rays fromthe converging light rays of the primar yoptic.

Chromospheric Features

Without a doubt, solar promi-nences are among the most beautiful andinteresting objects to observe in the heav-

ens. No other objects change size, shape,and brightness as do prominences. Theyare usually classified as quiescent (quiet)and eruptive (active), but all show activityand are evolving. Their lifetime can rangefrom minutes to weeks.

The classification system byMenzel and Evans (see below) groupsprominences according to whether theyoriginate in the corona or the chromos-phere, and if they are associated withsunspots. The names suggest the appear-ance of the prominence. No one classifi-cation scheme can describe all the phe-nomena observed.

As in the McIntosh sunspot sys-tem this is a three-letter designation classi-fication. The first letter will designate theplace of origin of the prominence, basicallywhether it is descending from the Coronaor ascending from the Chromosphere.The second letter whether the prominenceis related to a sunspot or not and the thir dletter is a description of the appearance ofthe prominence.

The delicate loops pictured onpage 6 would be classed as ASl. They were



A—PROMINENCES ORIGINATING IN CORONA (Descending)

S-(Spot Prominences) N-(Nonspot Prominences)

a. Rain a. Coronal rainf. Funnel b. Tree trunkl. Loop c. Tree

d. Hedgerowf. Suspended cloudm. Mound

B—PROMINENCES ORIGINATING IN CHROMOSPHERE(Ascending)

S-(Spot Prominences) N-(Nonspot Prominences)

s. Surge s. Spiculep. Puff

originating from above so as to bedescending (see arrows), hence the "A"classification. There is a spot just comingaround the limb with which these pr omi-nences were associated, therefore an "S" forthe second letter. Being in the form ofloops the third letter is to be "l". Thebright surge just to the nor th (right) of theloops would be classified as BSs.

Prominences are identical to thedark filaments seen on the solar disk. A tthe limb, they are viewed as bright emis-sion features against a dark sky. On thedisk, they are seen as dark absorption fea-tures against the bright sur face of the Sun.Filaments vary greatly in length and mayextend for more than a solar diameter.They have complex structures made up ofnumerous strands of gas, normally in a v er-tical pattern.

Gas condenses out of the muchhotter corona at the top of the pr ominenceand filters down to the chromospherebelow. Matter must be continuouslyreplaced to maintain the feature. Near thesurface, arch-like structures may dominategiving the appearance of feet anchored inthe chromosphere. On the disk, long fila-ments frequently display a smooth side anda scalloped side. The feet mark the loca-tion of a neutral line, or position where thepolarity of the longitudinal magnetic fieldreverses. The filaments are supported bymagnetic fields and are insulated from themuch hotter, but rarefied corona.

When a filament is formed nearan active region it may have a considerableextension in latitude. Due to differentialrotation it will become more inclined andgradually migrate to the poles. O ne fila-ment/prominence zone is about 10° pole-ward of the sunspot zones. It follows themigration of sunspots toward the equatorduring the 11-year solar cycle. Filamentsand prominences migrating to the polesform a second zone, which may form a"polar crown" of filaments on occasion.The fact that filaments are observed at all

latitudes enables astronomers to map largescale magnetic fields which supplementmagnetograms.

Filaments are intimately related toactive region evolution and respond vio-lently to solar flares and other magneticdisturbances. Some are observed to fadeand gradually disappear. Filaments associ-ated with active regions are often seenbreaking up and flowing into sunspotgroups. Apparently the magnetic field per-pendicular to the filament axis can nolonger support it and the field parallel tothe axis dominant.

Filaments are observed which sud-denly become active without warning andare erupted into the corona. As promi-nences their appearance is as a giant ar ch.The gas follows a helical path around thefilament axis and is connected to the chr o-mosphere at both ends. The great eruptiveprominence of June, 1946, which was fol-lowed out to 1.22 solar diameters, was afilament near the south pole of the S un.Disappearing filaments are a major causeof coronal transients and geomagneticeffects. It is common for a filament toreappear hours or days after its disappear-ance. The intensity of filaments variesfrom faint gray to black.

Most eruptive prominences areconfined to the latitudes of the sunspotzones. Their frequency follows the 11-yearsunspot cycle well. As with quiescentprominences, the descent of gas to thechromosphere is common in activeregions. A loop prominence begins with asmall spherical cloud condensing out ofthe corona above a sunspot group. Itbecomes elongated and breaks up to formstreamers forming both sides of the loop .Loop prominences have complex and deli-cate structures and are used to map mag-netic fields above sunspot groups. A giantloop display is particularly impressive as itmay consist of as many as tw enty separateloops and evolve over several hours. Activeloops are only associated with groups that



Fine loops and coronal rain from2015UT on April 1, 2001.

are good flare producers and have strongmagnetic fields. As observed at the limb,major flares often produce a tight systemof loops which grow rapidly. In less activegroups faint, single streamers and knots ofgas flow into active regions. This coronalrain is related to quiescent prominenceactivity.

Eruptive prominences have a vari-ety of shapes and sizes. Their lifetime aretypically less than 30 minutes. Surges,puffs, and sprays are closely associatedwith solar flares and centers of high activi-ty. A surge is normally a post-flar e phe-nomenon. Many relatively small flares arefollowed by an eruption. Observed on thedisk, surges appear as dark, elongated fea-tures which grow radially from a brightflare. Since they are moving rapidlytowards the observer, they will appear blueshifted and are best observed in the "bluewing" of the H-alpha line.

At the limb, surges may appear asbright spikes, or jets which grow rapidlyonly to fade away, or fall back into thechromosphere. Large eruptions begin asbright knots of gas which quickly expandand fragment into complex forms calledsprays. It is believed that eruptive promi-nences carry the chromospheric magneticfield with them. They can reach distancesof several hundred thousand km from thelimb. Observers should watch out forbright knots in the chromosphere thatappear suddenly. Also, observers shouldnot confuse prominence eruptions withsolar flares. Flares are more static and arenormally confined to the chromosphere inH-alpha. Flare rich active regions need tobe monitored in the blue wing of H-alphafor doppler shifted surges and sprays.

Solar Flares

The chromosphere near activeregions is an area of enhanced temperatureand brightness. Known as plages, theseemission features have strong vertical fields

up to 800 Gauss. Chromospheric plage isrelated to faculae in the photosphere. Hotcoronal condensations are observed overplages. Small, oval plage are often seen indeveloping active regions before sunspotsappear. Young plage is generally brighterand is associated with "arch filaments". Asa sunspot group matures the plage betweenthe preceding and following spots of abipolar group, becomes divided by a plagecorridor. The corridor coincides with theneutral line— the line that separates longi-tudinal magnetic fields of opposite polari-ty— where strong magnetic field occurs.Young compact groups with strong fieldshave thin, dark corridors. As groups decay,corridors become diffuse and disappear.Plages become fainter, fragmented andusually outline sunspots. In the K-line ofCalcium, plage appears more extensive andbrighter than in H-alpha. However, fila-ments and other absorption features arenot as well seen.

Of all the varied solar phenomena,solar flares are the most violent and inter-esting. A flare is a sudden release of energystored in the magnetic fields of activ eregions and then released in the form ofelectromagnetic radiation and atomic par-ticles. Radiation from flares covers theentire spectrum from x-rays to radio waves.The radiations at the ends of the spectr umfluctuate the greatest. We must, however,limit ourselves to the morphology of flar esas observed in the visual spectrum in thechromosphere.

A solar flare is a brightening of anexisting plage region. Intensity of flaresincreases rapidly, but takes longer to fadeto the pre-flare brightness. The time tomaximum intensity of a flare may be a fewminutes or even an hour. Most large,bright flares have a flash phase when arapid expansion and brightening occurs.

Solar flares have been classifiedaccording to area since the development ofthe spectrohelioscope. The classificationsystem used by the NOAA is as follows



Prominences seen against the solardisk (filaments) are seen in thisFebruary, 2003 image by HongKong amateur astronomer,Vincent Chan.

The changing appearance of thisfilament is evident in these twoimages obtained 05 August,2001. Gordon Garcia used a.3Å H-alpha filter on an APOwith an effective aperture of150mm.

1450 UT

1608 UT

and ranges from S, called subflares, to thelargest, called Importance 4. The additionof the sub-codes of F, N, or B are indica-tors of brightness (faint, normal, orbright). Examples: Importance 1B,Importance 3N and so on.

Flares are often measured in mil-lionths of a solar hemisphere, which is0.02 square degrees or 6.0 square secondsof arc. One second of arc on the Sun isequal to approximately 450 miles (730km). Due to foreshortening, flares nearthe limb are difficult to classify.

Twelve Items Concerning FlareProduction and Morphology

1. Solar Flares occur close to the neutrallines in active regions.2. Flares tend to occur near the same loca-

tion within a sunspot group.3. Large flares are preceded by smaller

flares and brightening of plages.4. Smaller flares appear as one or more

bright points of emission near spots oneither side of the neutral line.5. Large flares form parallel ribbons of

emission on both sides of a neutral lineand often touch sunspots.6. When new sunspots grow and interact

with older regions, flare productionincreases.7. More flares are observed during the

growth phase of sunspot groups, when

fields are rapidly changing, than during thedecay phase.8. Most flares are produced by type E and

F in the Zurich sunspot classification.9. Flare production is higher in magneti-

cally complex groups with a twisted neu-tral line. Simple bipolar and unipolargroups may not produce a single flare.10. Sunspot groups which are compact andhave spots of opposite polarity within onelarge irregular penumbra have the highestflare potential. Such groups are classed asDelta. Due to rotation of close spots suchgroups may violate Hale's Law of sunspotpolarity.11. Zones of higher flare activity can lastfor several rotations, or even years.12. Major flares are most frequent twoyears after sunspot maximum of the 11-year cycle.

Emergence of magnetic flux andsunspot motions appear to trigger flar es.Before a large flare a sheared neutral linewith high magnetic gradients is obser ved.Fibrils run parallel to the line. The fila-ment on the neutral line is destr oyed dur-ing the flare and becomes a flare spray.Several emission points brighten simulta-neously along the line. The flare strandmay split to form a Y-shaped flare. Theimpulsive flare is associated with bright hotkernels close to the neutral line. F lare ker-nels are related to the Helium D3 line flare



Optical Flare Importance Classification

Code Area-sqr. degrees Lifetime X-ray Class

S 2.0 or less < 4 min. C21 2.1 - 5.1 4 - 43 min. M32 5.2 - 12.4 10 - 90 min. X13 12.5 - 24.7 20 - 155 min. X54 > 24.7 .9 - 7.2 hr. X9

Subcodes : F = faint N = normal B = bright

and white light flares in the photosphere.Major flares that split into two ribbonsoften form an arcade of dark or brightloops between the strands. The separationof the ribbons coincides with the whitelight flare, X-ray and microwave bursts inaddition to the acceleration of par ticles.After the flare transition, arches (fibrils) areseen crossing the neutral line indicating arelaxed field.

H-alpha gives us only one pictureof a solar flare. The D3 helium line showsonly the intense kernels lower in the chro-mosphere. The H-alpha wing gives a simi-lar appearance. UV and X-ray photo-graphs from space show a much larger flarewhich begins in the corona. The hot X-raycloud is the source of particles (electrons)that penetrate into the chromosphere toform the H-alpha thermal flare. Onlylarge flares produce energetic particles thatpenetrate to the photosphere. Flares asstudied in the X-ray, microwave, etc. areimportant to the understanding of flar es,but are not within the scope of the S olarSection.

There is a type of flar e not directlyassociated with sunspots, called a Hyderflare. It is also known as a "DB" orDisparition Brusque. This flare is causedby a disappearing filament. These flaresare usually large and long lasting, but ar enot as energetic as spot flar es. DB flaresare associated with old decaying plages.

Possibly the most interesting, butalso difficult to observe flare related phe-nomenon is the Moreton Wave. It is ashock wave created by the explosive phaseand travels rapidly across the Sun. Thewave is bright in the H-alpha cor e, butdark in the wings of the line. I t induces adamped vertical oscillatory motion to fea-tures. Filaments will appear to "wink" inand out of view, due to doppler shiftingthrough the narrow passband of filters.Flare induced shock waves travel throughthe solar system.

It is particularly exciting to see an

area the size as the distance between theEarth and Moon flare up before your eyes!Large flares are quite a spectacle. Aroundsunspot maximum there are many timeswhen more than one flare and/or eruptiveprominence are occurring simultaneously.There is always great anticipation whensetting up your equipment, knowing thereis a large group visible. It is possible topredict the location of flares with goodaccuracy by studying active region mor-phology. Active regions may becomeactive or quiet without warning.

Chromospheric Background

The chromosphere, at the solarlimb, was described as early as 1875 b ySecchi. He likened its appearance to thatof a "prairie fire". The name chromos-phere or "color sphere" is from N.Lockyer. It is seen as a bright r ose coloredring just before and after totality of a solareclipse. The chromosphere is a thin, inho-mogeneous layer of gas above the photos-phere. It is only about 4,500 km in width.This layer appears to be composed ofnumerous jets of gas called spicules. Theaverage spicule is 1,000 km in diameterand from 6,000 to 10,000 km in length.A spicule rises out of the chr omosphere,then falls back or fades in about 5 to 10minutes. Most are vertical but inclinationsof 20° are observed at times. Spicules aregenerally larger at the Sun's poles and mayvary with the sunspot cycle.

Viewed in the core of the H-alphaline, the solar disc shows a large scale gran-ulation network. The individual cellsaverage 30,000 km (40 seconds of ar c) indiameter and have a lifetime of about oneday. This supergranulation is easier toobserve than the 1-2 arc second photos-pheric granulation. The center of a cell isbrighter showing faint mottling in smallertelescopes. At the boundary of a cell darkhair-like features are observed. Theseabsorption features on the disk are identi-



cal to the spicules seen in bright emissionon the limb.

Rosettes, chains, and brushes arenames of patterns that spicules assume onthe disk. Rosettes are groups of spiculesthat radiate out of a common center.Chains are rows of spicules. Brushes aregroups of spicules observed near the limbthat give the Sun a "hairy" appearance.These patterns are best seen in the wingsof the line. Enhanced magnetic fields arefound in the supergranual boundaries. I nthe K-line of Calcium, the chromosphericnetwork is well seen, but the cells ar e darkand the boundaries are bright. Indeed, inhigh resolution H-alpha photographsbright points are found at the bases ofspicules indicating enhanced temperature.

Near active regions spiculesbecome inclined and enlarged due to theinfluence of magnetic fields. Known asfibrils, these features follow the transversemagnetic fields parallel to the solar sur face.They are typically 11,000 km long and725-2,200 km wide. Fibrils connectregions of opposite polarity and giv e thechromosphere a complex appearance nearactive regions. In bipolar groups fibrilsgive the appearance of iron filings in amagnetic field. They align themselvesradially from large sunspots forming anextension of the penumbra, called thesuperpenumbra. They extend out to a dis-tance of about a spot diameter. In largegroups a much larger area of nearly radialfibrils are observed which may cover 10%of the visible disk, called the solar vortex.A special type of fibril which is obser vedwhen an active region is growing is thearch filament. Arches are dark, thick fea-tures that are observed in young plage.Fibrils and arch filaments are low lying fea-tures and are not well observed at the limb.An arch filament system may last thr eedays, but individual filaments only last 10-30 minutes. The neutral line usuallybisects an arch filament system. After asunspot group matures the arch filaments

are replaced by fainter field transition arch-es.

The fibrils near quiescent fila-ments align themselves by a small angle tothe axis of the filament. I n the blue wingof H-alpha, featureless bands are oftenobserved where filaments exist. These fila-ment channels are observed before a fila-ment forms and remains visible after it dis-appears.

Moustaches, also called EllermanBombs, are small bright points that ar ethree seconds of arc or less in diameter.They are best seen in the wings of the lineand occur near sunspots. S imilar featuresare found at the bases of spicules andactive filaments. Ellerman Bombs may lastfrom a few minutes to several hours andare not generally related to flares.

A great deal of solar detail is visi-ble at the resolution of 1 second of ar c,much more than the observer can drawreliably. A 6-inch (15 cm) aper ture canresolve to one second of ar c in good seeingand a 4-inch (10 cm) will sho w the chro-mospheric fine structures well. Smallerapertures can be used effectively on moredays than larger ones due to atmosphericseeing limitations. Since the major fea-tures are 5 seconds of arc or greater, a largetelescope is not required to observe these.

Observing Hints

The best choice of telescope to usefor solar observing in general, and formonochromatic observing in particular, isa refractor. A Newtonian telescope is notrecommended for monochromatic work.On average they have shorter focal lengths,central obstructions, and the access to thefocal plane outside the tube (focus trav el)ordinarily limits the use of filters and cam-era attachments. Complete solar telescopesare marketed today which are excellentinstruments for monochromatic observing.They come in a variety of apertures andprice ranges suited for many amateurs



from the novice to the veteran observer. A prominence telescope or filter-

ing device which uses an occulting diskwill necessitate a good driving mechanismfor both axles of the equatorial mount. I fa heliostat or coelostat is used to feed a sta-tionary telescope, slewing controls will beneeded.

Seeing condition information asdiscussed in the white light edition of this

andbook will apply also here. Domes,while fine for windy mountain tops, con-tribute to poor local seeing and ar e bestavoided. An open air situation with mini-mal heat traps (such as surrounding build-ings, asphalt or concrete drives) is better.Observatories with a roll-off or split roofare preferred. Study your local weatherpatterns and learn when the seeing is goodand be prepared to observe at those times.Often morning is good before local heatingdestroys seeing. Watch high pressure areasand cold fronts, then note how they affectyour seeing conditions. Sky transparencyis important when observing limb phe-nomena. Hazy skies with a lot of scatterreduce the contrast of prominences makingthem increasingly difficult to observe.

Observers should try using an eyeshield or head cover to eliminate scatteredsunlight at the eyepiece. The monochro-matic view is usually fainter than a whitelight, and a shield will help adapt the ey eto seeing fainter detail.



These samples illustrate the various means our observers havechosen to place the technical information within their images.What is impor ant is that the questions what, when, who,and how are addressed. We cannot stress enough the attentionneeded to include this information with all observations sub-mitted. Refer to the photography form for requested informa-tion.

h

The upper four images illustrate several forms of limb activity. The three on the left are surge prominences of thespray, jet, and mound variety. A classic loop system is on the right. The lower grouping was obtained 22 August,2001 during Rotation 1979 depicting the rapid development of a surge at 60 second intervals beginning at 1505

UT. J. Jenkins observed with a 125mm f/18 refractor and a 10Å H-alpha filter.

Flares and filamentary detail near a sunspotgroup on 18 August, 2001. Image by Vic andJen Winter with a .4Å H-alpha filter on a4.75" singlet red glass lens operating at f/30.

Fine detail of the gas suspended above the solar limb in thisGordon Garcia image from 14 May, 2000.

Suggested Reading List

(Ed.note—while no list can be complete because it will be out of date yearly as new books are published, this list-ing will satisfy most readers and supply sources to answer many questions not addressed in this Handbook.)

Books of Primarily Historical Interest

Abbot, C.G., THE SUN, D. Appleton & Co., NY, 1912Abetti, G., THE SUN, Macmillan Co., NY, 1961Abetti, G., SOLAR RESEARCH, Macmillan Co., NY 1963Baxter, W.M., THE SUN AND THE AMATEUR ASTRONOMER, Drake Publ. Inc., NY, 1973Ellison, M.A., THE SUN AND ITS INFLUENCE, Macmillan Co., NY, 1955Kuiper, G.P., editor, THE SUN, University of Chicago Press, Chicago, 1953Meadows, A., EARLY SOLAR PHYSICS, Pergamon Press, 1970Menzel, D.H., OUR SUN, Harvard University Press, Cambridge, MA, 1959Mitchell, S.A., ECLIPSES OF THE SUN, Columbia University Press, NY, 1935Moore, P., THE SUN, Norton, NY, 1968Newton, H.W., THE FACE OF THE SUN, Penguin Books, London, 1958Pepin, R.O., THE ANCIENT SUN, Pergamon Press, NY, 1979Proctor, M., ROMANCE OF THE SUN, Harper & Bros. Publishing, NY, 1927Stetson, H.T., SUNSPOTS IN ACTION, Ronald Press, NY, 1947Thackery, A.D., ASTRONOMICAL SPECTROSCOPY, Macmillan, NY, 1961Young, C.A., THE SUN, D. Appelton & Co., NY, 1898Zurin, H., THE SOLAR ATMOSPHERE, Blaisdell Publishing, Waltham, MA, 1966

General Interest Reading

Giovanelli, R.G., SECRETS OF THE SUN, Cambridge University Press, NY, 1984Lang, K.R., THE CAMBRIDGE ENCYCLOPEDIA OF THE SUN, Cambridge University Press, NY, 2001McKinnon, J.A., SUNSPOT NUMBERS: 1610-1985, World Data Center, Boulder, CO, 1987Nicholson, I., THE SUN, Rand McNally, NY, 1982Noyes, R.W., THE SUN, OUR STAR, Harvard University Press, Cambridge, MA, 1982Pasachoff, J.M., THE COMPLETE IDIOT'S GUIDE TO THE SUN, Alpha, NY, 2003Waldmeir, M., THE SUNSPOT ACTIVITY IN THE YEARS 1610-1960, Zurich, 1961

Novice/Intermediate/Advanced Reading

Beck, R., SOLAR ASTRONOMY HANDBOOK, Willman-Bell, Richmond, VA, 1988Bray, R.J./Loughhead, R.E., SUNSPOTS, Dover, NY, 1964Bray, R.J./Loughhead, R.E., THE SOLAR CHROMOSPHERE, Dover, NY, 1974Bray, R.J./Loughhead, R.E., THE SOLAR GRANULATION, Chapman & Hall, London, 1967Brody, J., THE ENIGMA OF SUNSPOTS, Floris Books, Edinburgh, Scotland, 2002Cram,L.E./Thomas, J.H., THE PHYSICS OF SUNSPOTS, Sacramento Peak, Sunspot, NM, 1981Espenak, F., FIFTY YEAR CANON OF SOLAR ECLIPSES 1986-2035, NASA, Washington, D.C., 1987

Novice/Intermediate/Advanced Reading (cont.)

Foukal, P.V., SOLAR ASTROPHYSICS, Wiley & Sons, NY, 1990Gibson, E.G., THE QUIET SUN, NASA, Washington, D.C., 1973Henderson, S.T., DAYLIGHT AND ITS SPECTRUM, Halstead Press, NY, 1977Jenkins, J.L., THE SUN AND HOW TO OBSERVE IT, Springer-Verlag, NY, 2009Kippenhahn, R., DISCOVERING THE SECRETS OF THE SUN, Wiley & Sons,NY, 1994Kitchin, C., SOLAR OBSERVING TECHNIQUES, Springer-Verlag, London, 2002Kitchin, C., OPTICAL ASTRONOMICAL SPECTROSCOPY, IoP Press, 1995 Macdonald, L., HOW TO OBSERVE THE SUN SAFELY, Springer-Verlag, London, 2003Neidig, D.F., THE LOWER ATMOSPHERE OF SOLAR FLARES, Sacramento Peak, Sunspot, NM, 1981Phillips, K.J.H., GUIDE TO THE SUN, Cambridge University Press, NY, 1992Sawyer, R.A., EXPERIMENTAL SPECTROSCOPY, Prentice-Hall, 1946 (Dover, 1963)Spence, P., SUN OBSERVER'S GUIDE, Firefly Books, Richmond Hill, Ontario, 2004Stix, M., SUN, Springer-Verlag, London, 1991Strong, C.L., THE AMATEUR SCIENTIST, Simon & Schuster, NY, 1960Sturrock, P.A., editor, SOLAR FLARES, Colorado Assoc. University Press, Boulder, CO, 1980Sturrock, P.A., editor, PHYSICS OF THE SUN, D. Reidel Publishing, Dordrecht, 1986Svestka, Z., SOLAR FLARES, D. Reidel Publishing, Dordrecht, 1976Tandberg-Hanssen, E., SOLAR PROMINENCES, D. Reidel, 1974Taylor, P., OBSERVING THE SUN, Cambridge University Press, NY, 1991Taylor, P./Hendrickson, N., BEGINNER'S GUIDE TO THE SUN, Kalmbach Books, WI, 1995Veio, F., THE SUN IN H-ALPHA LIGHT WITH A SPECTROHELIOSCOPE, Veio, 1991White, O., THE SOLAR OUTPUT AND ITS VARIATION, Colorado University Press, Boulder, Co, 1977Xanthankis, J.N., SOLAR PHYSICS, Wiley & Sons, NY, 1968Zurin, H., ASTROPHYSICS OF THE SUN, Cambridge University Press, NY, 1968

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