Association of Lunar & Planetary Observers …...Association of Lunar & Planetary Observers Association of Lunar & Planetary Observers Guidelines for the Observation of White Light

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



































Guidelines for theObservation ofWhite LightSolar 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

Rik HillLunar and Planetary Labratory

University of Arizona

Established 1947

Guidelines for the Observationof White Light Solar Phenomena

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

Edited by

Jamey JenkinsAsst. Coordinator, ALPO Solar Section

Originally compiled by

Rik HillLunar and Planetary Labratory

University of Arizona

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

About Solar Observing.....................................................................................1

White Light Observing .....................................................................................7

Visible Features......................................................................................13

White Light Flares..................................................................................23

Suggested Reading ........................................................................................27

Solar Observing in General

The late Jim Loudon a space sci-ence and astronomy lecturer with theUniversity of Michigan, had about the bestdefinition of a star, and the Sun is a typicalstar. He said, "The Sun and any star forthat matter is a very big, very hot, ball ofglowing gas."

"Brevity is the soul of wit!" saysShakespeare, and Jim was a witty fellow.But this simple sentence says an awful lot,accurately defining the very essence of theSun. The Sun though an average star is stillenormous with a diameter of 1.4 millionkilometers. It has a "surface" temperatureof 5600K, but in the center is o ver fifteenmillion kelvins while in the corona, aroundthree million kelvins. The kelvin is a tem-perature measurement system based onabsolute zero. For conversion purposes,degrees Celsius + 273.15 equals kelvins.Though many have tried for the last fourhundred years no one has shown the Sunto be other than nearly spherical withinerror of measurement.

The Sun does not burn, and thisterm should never be used as it is v ery mis-leading, especially to the general public.The gas of the Sun glows from the energyof fusion turning hydrogen into helium.Lastly, the Sun is gas throughout. Granted,at the pressures and temperatures in thecenter it would not seem like it, but her etoo it is a gas, in plasma form.

So Loudon's definition covers themain points pretty well, and it's easy toremember!

The Sun is not generally viewed inits proper perspective among objects in theuniverse. When we speak, we often speakof THE Sun. But do we say THE Jupiter, orTHE Mars? Nope. We find that the Sunwas placed in its own category in antiquitywhen its relationship to the stars wasunknown; like THE stars, THE planets, THE

galaxies, and THE Moon. The result? Thiscauses most amateur astronomers to see

solar studies as a separate discipline fr omreal astronomy. As such solar astronomersshare a special comraderie among theirbrethren solarphiles.

Why do we amateurs study theSun? Beyond the obvious responses ofbeing the life giver to the planet or theclosest placed star, the answer may befound in the shear dynamics of the S un!An ever changing view given the observer,in some cases on time scales of a fe w min-utes, can be an attractive alternative to thealmost static views presented elsewhere inthe sky. Sunspots grow and decay, flaresburst, prominences erupt or rain onto the"surface" below; each day presents a newface to the Sun. Solar observing can befun, exciting, and forever interesting. Nowyou understand, why we amateurs studythe Sun.

Solar observing, while attractive tothe astronomer for those reasons does how-ever present a few obstacles that the typicalnight sky observer doesn't face. Some ofthese hurdles include safety, local seeingconditions, and suitable instrumentation.The amateur astronomer, by cleverlyaddressing these obstacles, we are confidentwill find it possible to make useful obser-vations, and thereby contribute to theknowledge of the nearest star, THE Sun.

Safety and the Sun

Unless special equipment or tech-niques are in use, any observer lookingeven the shortest amount of time at theSun through an optical instrument willforever damage his eyes! This is a lessonthat must be "burnt" into the brain of allpotential solar observers from the begin-ning.

Here are two examples that willhelp make the point to ev eryone con-cerned. First, just hold a piece of paper inthe focal plane of any telescope focused onthe sun. What occurs? The paper isinstantly ignited from the heat of the Sun!

About Solar Observing

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

Secondly, the United States AirForce in the last century conducted a seriesof experiments with rabbits and the effectof nuclear explosions to their visual senses.Rabbits have similar eye structure to thatof humans. From a distance of fifty milesthe fireball of a twenty megaton bombburned a hole in the r etina and vaporizedthe liquid surrounding the iris of the rab-bit's eyes. Bombs of this siz e do not gen-erate even the smallest part of the energyproduced by the Sun in a billionth of asecond! The dangers involved in solarobserving can never be overemphasized.

On the other hand, a car efulapproach and proper knowledge allows oneto safely observe the Sun without reserva-tion. The safest method of white lightobservation is the projection method bywhich the Sun's image is viewed indirectlyafter it has been projected onto a whitescreen by the telescope.

Another safe method is theHerschel Wedge (a special prism) with sec-ondary filters at the telescope ey epiece.Objective filters having special reflectivecoatings on a glass or mylar material ar ethe most often used appliance.

One solar filter to warn aboutusing as there may still be some on themarket is the so-called "solar ey epiece fil-ter" that comes occasionally with a lo werend department store telescope. Thisdevice is intended to screw into the base ofan eyepiece which is then inser ted into thetelescope focuser. The filter is no morethan a dark green "welder's glass" whichdoes indeed darken the Sun's brightness.While the transmission of infrared light isretarded with these filters, serious danger isthe possible shattering of the filter fromfocused heat at the telescope's focal plane.Shattering takes but a moment and thenthe eye is exposed to the blinding light andheat of the Sun. The wise and cautioussolar observer will discard this type of filterimmediately.

Seeing Conditions and the Sun

Veteran solar observer GordonGarcia once noted that, "seeing is ev ery-thing!" What Gordon was referring to wasthe blanket of turbulent air that surr oundsour planet and how it effects our view ofobjects in space.

Observing celestial bodies hasbeen compared to looking out at the worldfrom the bottom of a swimming pool. I nthe same way that the inter vening waterdistorts our view of the people and objectsat the poolside, the atmosphere playshavoc with our view of the stars, planets,and all celestial bodies, including the S unas we look out into space from the surfaceof the earth. In solar observing this is evenmore pronounced as the Sun tends to heatup the air and our surroundings resultingin even greater turbulence between youand the outside universe.

While the upper atmosphere cancause seeing problems, the worst distur-bances appear to occur closer to theobserver. Indeed, some authorities claimthat as high as 90% of the disturbances tothe atmosphere can be traced to the first100 meters of air above the ground!

In solar observing, the ability tosee 1-arc second detail or better is desir edfor the best work. It is estimated that suchoccurs only 1% of the time. This meansthat astronomers have had to adopt pro-grams that can take advantage of these raremoments of fine seeing. The past methodmost commonly used with professionalswas to have an automatic film camera pho-tograph at regular intervals (once a minuteor less) and then just weed out the badimages. A modification of this was to hav esome sort of atmospheric monitor that willtrigger a shutter when it detects seeing ofgood quality. These are costly and com-plex techniques. Amateur astronomerscould make a significant contribution inthis field by adopting these plans and sim-ply visually monitoring the seeing and


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

imaging only when those moments of bestseeing occur; in effect, becoming the mon-itoring system yourself.

Modern digital media allow theacquition of hundreds of images for pen-nies on the dollar compared to older filmtechniques. This means it is now cost feasi-ble to literally ignore monitoring of seeingconditions, shoot blindly and capture atleast a few moments of fine seeing. Maybe.

Improving Local Seeing

Horizontally, seeing is drasticallychanged by the lay of the land, includingman made structures. Even at major obser-vatory complexes, chosen for their excel-lent seeing, the quality of the sky on anygiven night can vary from excellent at onetelescope to unusable at another. Daytimeseeing can particularly vary like this.Observers should be aware of factors thatcan better their chances of havingimproved seeing conditions.

Avoid buildings anywhere near, orbeneath, the path between the telescopeand Sun. This is especially important withthe common metal garden shed often usedas observatories by the amateur observer.Domes will be a problem no matter whatthe observer does. The heated metal aboutthe slit will disturb the air. In fact, manyastronomers consider observatories placeswhere bad seeing is created and retained.Ideally some sort of open air observatoryor one where the optics are in the open airfeeding an image into an enclosure are tobe sought out.

Observers should take precautionswhen locating an observatory, making surethat nothing is blocking the pr evailingwinds at their site. Such obstructions areknown to increase turbulence.

Many night sky telescopes haveflat black surfaces that face skyward. Forthat kind of observing this is not a prob-lem. But for daytime observing the unhitting anything flat black in or ar ound the

telescope should be avoided at all costs. Allsurfaces that face the un should be white.Tests done at Kitt Peak in the 1970'sshowed significant heating as soon as anycolor other than white was used, thisincluded polished aluminum. Even an offwhite color was markedly warmer whenexposed to sunlight than pure titaniumwhite. Only surfaces that are seen by theeyepiece should be flat black. I f you con-struct your own solar telescope, you willfind it highly advantageous to use a largerthan necessary tube with an aper ture stopslightly bigger than the primary optic (lensor mirror) to keep sunlight off the interiorwalls of the telescope. This will help reducedisturbing air currents within the tube.

With such considerations in mindduring construction you will find that evena modest 4-inch telescope specially builtfor solar observing will many times outperform a similar aperture night sky tele-scope adapted to solar observing.

Does All Bad Seeing Results From Heat?

If you have ever looked with a tel-escope at a rising planet or star in thenight sky you have undoubtedly beenstruck by the little spectrum they form asthey rise through the thick layer of atmos-phere near the horizon. This prismaticeffect of our atmosphere is called atmos-pheric refraction. As soon as you get morethan 25 degrees away from the zenith, thisrefraction becomes greater than one arcsecond. There is a way around this deterio-ration: filter the view, so you observe inonly one color.

Even broad band filters, like thoseused in planetary observation will improvethe situation. But the narrower the band-width of the filter, the lower in the sky youcan achieve a potential resolution of arcsecond quality. This is a very importantfactor in photography.

Filters are also used to enhance thecontrast of selected features and to a lesser



S

S

degree improve the performance of somerefracting telescopes.

Quality of Seeing in Daytime Skies

With the Sun there are several see-ing quality indicators. When the seeing iswhat we call better than one ar c second,granulation will be clearly resolved. If theseeing is on the order of 1 to 2-arc secondsthe appearance of granulation is mottled.From 2 to 5-arc seconds the granulation isonly occasionally as good as mottled andany pores will pop in and out of vie w. Anyseeing condition experienced worse thanthis is considered to be useless.

Sometimes observers use descrip-tive terms to deliniate the quality of theatmosphere. For instance, "fair" could rep-resent 2 to 5-arc second seeing. “Poor”could be defined as worse than 2 to 5-ar cseconds. When reporting observations tothe Solar Section indicate the quality ofseeing by arc seconds rather than the pure-ly subjective descriptions of poor, fair,good, etc.

Study Local Observing Conditions

By studying the Sun at varioustimes throughout the day, under a varietyof weather conditions and noting therelevent air quality, you may discover cir-cumstances in which your local seeing isbetter than average. Perhaps your best see-ing occurs early in the morning, befor e theSun has had a chance to heat up localrooftops; or at mid afternoon with the S unhigh in the sky and a light br eeze from theeast or west. Some observers find lateafternoon to bring conditions that allowfine detail to be seen.

The point here is to pay attentionand experiment with your observing sched-ule. Many sites will have an optimumtime of day and set of conditions that willfavor solar observing. Study your site anddiscover those times and conditions.

By applying suggestions gleanedfrom this booklet you can maximize theuse of your observing site to produceresults that rival observations made at pro-fessional observatories.

A Telescope for Solar Observing

Any telescope may be used forobserving the Sun so long as the vie w is asafe one. Most serious observers however,have through experience determined thatin the amateur ranks and with the sev eraldesigns available, the refracting telescope isbetter suited for most types of solar obser v-ing.

For instance, consider that com-pound telescopes such as the Maksutov orSchmidt-Cassegrain are not well suited forsolar projection because they are suceptibleto internal heat damage. The refractor withits unobstructed light path does not havethis issue. An unobstructed path also facili-tates maximum contrast of low contrastsolar features. Why? Because more lightwill become concentrated in the Air y discand not in the surrounding diffractionrings. The secondary mirrors of a Mak orSCT disrupt the ideal Airy pattern. Thesame is true regarding a classic Newtonian.

Mechanically, the refractor alsohas plenty of back focus to easily adapt thenecessary appliances for monochromaticobserving (amplifiers, end-loading filters,tilting mechanisms, etc.).

An Optimized Telescope

Often an observer will constructor modify an existing telescope for strictlyobserving the Sun. Here are a few thoughtsto keep in mind when cr eating a telescopeintended for solar observations.

A primary consideration in design-ing a solar telescope is that ther e should beas few optical elements as possible in theinstrument. In solar observing, imageabberations, scattered light and heating of



optical components are extremely damag-ing to the quality of the final image. M anysolar features are of low relative contrastand such effects tend to easily "wash-out"the view.

A further point to keep in mind isthat the more optical components in a sys-tem, the quality of each should be at leastequal to the desired wavefront within thesystem. Wavefront is defined as the imagi-nary surface representing the correspon-ding points of a wave that vibrate in uni-son—in this case light waves transversingthe telescope's optical path. The delicatedetails of the Sun require optics capable ofdelivering near their theoretical limits. Thisresults in greater expense and fabricationtime for each component. Therefore ourgoal in a dedicated solar telescope ought beto keep it simple, while deliv ering sharpand contrasty images.

The instrument should be capableof resolving to 1-arc second. This meansthe aperture must be at least fiv e inches(125mm) diameter, which is not meant tosay that useful work can’t be done withless. If an observer has a smaller telescopethey should make the best of it rather thanputting off work until the ideal telescope isobtained! In the past apertures of 1.6-inches (40mm) have been used to someadvantage in various observing projects. Asdiscussed earlier, with apertures larger than4 or 5-inches a telescope's per formancewill on occasion be limited by local seeingconditions. But then again let this not be adeterrent to observing. Daytime seeing canbe notoriously bad. Quite often the prob-lem is the heating of instr uments, sur-rounding buildings, or some other detri-mental local seeing effect. Because of thisthe maxim was coined that 1-ar c secondseeing is achieved only 1% of the timeduring daylight hours, even at the verybest of sites. So apertures greater than 4 or5-inches would be sky limited nearly 99%of the time! Ground based instruments of6-inches aperture and greater perform

essentially the same most, but not all of thetime.

To summerize, whether you adapta conventional night sky telescope or pur-chase/assemble a dedicated solar tele-scope—the key to repeatable success isfound in simple, well-made medium aper-ture optics used in conjunction with a spe-cific methodology to your observing pro-gram.

How to Locate the Sun in a Telescope

Ordinarily one wouldn’t think thatfinding the Sun through a telescope wouldbe a challenge, it is easily visible in the sky .Safety though is our concern, and sightingalong the telescope tube is just as danger ousas looking at the Sun through an unfilteredfinder.

The usual means of locating theSun with a telescope is to watch the tele-scope's shadow on the ground. When thetelescope it pointed at the Sun, this shad-ow will be smallest.

Several telescope manufacturersproduce pinhole solar finders that ar esuperb for locating the Sun. To preventinjury we suggest any optical finder onyour telescope be removed, or at leastcapped when doing solar observing.



White Light Solar Observing

By definition, white light observ-ing means that you are seeing the Sun inthe combined colors of light from the solarspectrum. This can also be known asobserving in the solar continuum.

In white light the Sun appearsmuch as it does in the sky. The projectionmethod of viewing yields the truest colorrendition; while an over the objective filtertypically offers a white, orange or blue castto the Sun. The view through a HerschelWedge is dependent on the color of anysupplementary filters located between thewedge and the eyepiece.

White light observing affords thesolar observer an economical means intothe realm of daytime astronomy. Narrowband observing, such as in hydrogen alphalight can be an expensive undertaking forthe amateur, the cost maybe becoming adiscouragement. It is however possible to"tool up" a telescope for white lightobserving with the addition of just a singleinexpensive accessory, be it a projectionscreen or a white light objectiv e filter.

The layer of the Sun that we see inwhite light is called the photosphere. Themost obvious features located here aresunspots. Sunspots can be large or small,singular or in clusters known as groups.Sunspot groups grow in complexity thendecay with time. The white light observerwill also view faculae, extensive vein likepatches seen near and often encompassingsunspot groups. Solar granulation is visibleacross the entire photosphere when the see-ing is steady.

Granulation gives a textured ororange peel appearance to the solar disc—it is the combined effect of individualgranules—each being the top of a columnof gas rising from the convective layer ofthe Sun. Features called light bridges maydivide sunspot umbra and are quite com-mon, unlike the rarer white light flares(WLFs).

Morphology or the recording ofthe changing appearance of the Sun and itssunspots is the main focus of white lightobservers in the ALPO Solar Section. Weencourage observers to record sunspotgrowth/decay via paper and pencil sketch-ing or by photography through the use ofmodern digital imaging techniques. I t ispossible for the amateur to make signifi-cant contributions to the acquisition ofdata regarding solar morphology. Qualityobservations are in demand that do notrequire sophisticated equipment, but willrequire a compelling interest on the part ofthe observer, and careful attention todetail.

Appliances for Solar Observing

Many who begin white lightobservations modify night sky telescopesfor solar observing. Unlike the typical astrotelescope designed to gather light, the solartelescope must reject almost all of the lightthat falls on the aper ture. The means ofachieving this light rejection are discussedin the following pages, but again alwayskeep in mind that: solar observing is theonly inherently dangerous observing anamateur astronomer can do. If you takeone false step, one unsafe procedure, itcould and likely will result in irreversibleblindness.

But as stated earlier— with propercare and knowledge one can safely observethe Sun without the slightest amount offear or reservation.

Solar Projection

The safest method of observingthe Sun is the projection method. Withthis arrangement an eyepiece is inserted inthe telescope's focuser to project an imageof the Sun onto a white screen. No filtersare used, but safety is assur ed because theSun is viewed indirectly. The analogy of aslide projector comes to mind with the

White Light Observing


solar image formed by the objective servingas the slide and the eyepiece as the projec-tion lens. It's easy to see that the fartherthe screen is from the eyepiece, the largerbut dimmer is the projected image of theSun.

Refractors and most Newtoniantelescopes are both suitable for solar pro-jection but it is wise to avoid a compoundtelescope such as the Maksutov orSchmidt-Cassegrain since heat from theSun can damage internal components suchas baffle tubes. Heat may also damageexpensive multi-element eyepieces by melt-ing cement that is used between lenses.Older Huygens or Ramsden designs areokay since they contain no cemented ele-ments. These eyepieces usually can bepicked up second hand, purchased fromsome optical suppliers, or even assembledat home from standard lenses.

Projection is done onto a flatwhite card stock or bristol board material.The key when using solar projection isproviding a shaded environment for theprojection screen, so that the contrast ofsolar features is not compromised. Manyclever observers have adapted various appa-ratus ranging from a light weight woodenbox to an oatmeal carton to serve as the"shade" for the screen.

We recommend the use of aportable observing box called the HossfieldPyramid. The device is simply a "pyramid"shaped box many times constructed ofsturdy cardboard or thin wood. The smallend of the box attaches to the projectingeyepiece and the viewing screen is locatedat the base of the pyramid. Paint theinside of the box flat black and makeallowance for an access window on oneside to permit the observer to see the pro-jection screen.

Solar projection is an ideal meansfor obtaining whole disc drawings of theSun. A grid system may be incorporatedon the screen for transferring spot posi-tions to the recording form or the form

itself may be attached to the projectionscreen and spot positions gently drawn onit. How sturdy an outfit one has, and howeasily the observer can access the screen arethe determining factors for efficiency.

Calculating projection distancesand the diameter of the solar disc may beperformed on the basis of this simple for-mula: A = B x (C / D). The desired diame-ter of the projected solar disc is A; thediameter of the sun at the telescope's focalplane is B (calculated as approximately1/100th of the focal length of telescope); Cis the distance from eyepiece to screen; andD is the focal length of the projecting eye-piece. See below.

This formula while not absolutelyperfect owing to the varying size of thesolar disc throughout the year is accurateenough to design a system that permitsminor adjustments to the projection dis-tance. A disc diameter of 18 cm (approxi-mately 7-inches) is the preferred size.Alternately, an observer could by trial anderror hold a piece of stiff white paperbehind the eyepiece, and then by varyingthe distance to the paper and refocusingthe eyepiece find the necessary separationsuitable for the desired projection diame-ter.

Figures 2 and 3 illustrate a simplehomemade Hossfield Pyramid made ofblack foam board, white card stock, sprayadhesive, and masking tape. Attached tothe star diagonal, the open end of thepyramid is always in the shade providing abright contrasty view of the solar disc andwhite light features.


8 © 2010 Association of Lunar & Planetary Observers - All rights reserved.

Herschel Wedge

The Herschel Wedge is a thinwedge shaped, unsilvered prism with opti-cally flat surfaces. John Herschel developedthe concept long ago and several commer-cial units are found on the market today.By reflecting a small percentage of theSun's light to the eyepiece the Sun’s inten-sity is reduced dramatically.

Note— A HERSCHEL WEDGE IS

NOT SAFE IF USED ALONE— additional fil-tration is required between the prism andthe eyepiece to provide safe viewing.

In use, a Herschel Wedge replacesthe diagonal on the telescope. For the samereasoning that solar projection is not asuitable option for a compound telescope,a Wedge is not suitable either. Heat willbe present within the optical tube assem-bly, heat that could damage internal com-ponents. A Newtonian reflector is adapt-able, but again best suited for a H erschelWedge is the refracting telescope.

With a refractor light passingthrough the objective strikes the front sur-face of the Wedge, approximately 5% ofthis light is reflected to the eyepiece. Theremaining 95% of light passes through thebackside of the wedge (a need for protec-tion from burns here) and a small por tionis absorbed by the prism itself.

Since the Sun is still nearly 5% ofits normal brightness, additional filtrationis required. Many observers employ neutraldensity (ND) filters with a value from .6 -3.0 ND. Sometimes a linear polarizing fil-ter is placed in the filter pack betw eenprism and eyepiece to permit minuteadjustment of the Sun's brightness.Colored glass filters, as in planetar y observ-ing may also be used to r eplace or supple-ment the ND filters. Colored filters havethe advantage of enhancing contrast of cer-tain white light features visible in the pho-tosphere.

Lastly, between the Wedge andeyepiece, as a "safety buffer" we recom-

mend the use of an infrar ed (IR) rejectionfilter. Ultimately, always follow the manu-facturer's guidelines in the use of theWedge, but the addition of a good IR filtercan only protect your eyes, and not harmthem.

Experienced solar observers havesaid that the key to using a H erschelWedge is allowing the telescope's compo-nents to warm to a point of temper tureequilibrium. Once the optics and air with-in the tube have settled down, the view issuperb, often being limited only by atmos-pheric conditions.

Objective Filters

A number of external solar filtersexist on the market that reduce the lightand heat entering your telescope. Theamount of light you reject will depend onthe type of work you intend on doing.High resolution photography generallyrequires a thinner density filter (2.5 - 4.0ND) to allow more light to pass than isrecommended for visual use. Objective fil-ters intended for visual use ar e somewhatdenser being rated at a neutral density ofabout 5.0.

At the time of this writing ther eare two basic types of objectiv e filters avail-able. These are the aluminized flexible-filmtypes (usually an optical grade mylar) andthe metal coating on glass types. I f madeproperly and of high quality, both workexcellently and for general observing nei-ther is to be preferred over the other. Themylar-type are coated with aluminum andoften produce a blue-white or neutralimage of the sun. Glass filters are usuallycoated with a metal film (i.e. I nconel) andwill produce an orange or nearly neutralview. Infrared and ultraviolet radiationshould be removed with either type.

Try out a variety of filters beforeyou buy one of your own. Ask for opinionsfrom other observers and then maybe pur-chase smaller sizes of several brands/types



Fig. 2

Fig. 3

to use (off axis on a r eflecting telescope oron axis with a refractor) until you canmake up your own mind on a preference.

When you get beyond generalobserving, factors to consider with the pur-chase or assembly of an objectiv e filterinclude: the peak transmission of the wav e-length it passes (that is, what color the S unappears through the telescope), the aper-ture and consequently the resolution onedesires, and the cost involved. Some filterspass more light from the blue end of thespectrum giving the sun a bluish-whiteappearance. In white light observing thiswill favor observing faculae, granulation,or white light flares. Filters that transmitmore so toward the red spectral regiontend to perform well on sunspot detail, butfacular views become diminished.

An excellent choice is an objectivefilter giving a neutral (basicly white) vie w.With the addition of broadband (planetarytype) filters at the eyepiece to this filter itbecomes possible to selectively enhance thecontrast of those features you wish tostudy.

It is vital to understand that whena glass or mylar filter is placed in fr ont ofan objective, it becomes a par t of the opti-cal train of that telescope. The quality ofan objective filter is indicated by thedegree of distortion (or wavefront error) itimparts to the telescope. Mylar-type filtersalways hinder the performance of a tele-scope to some degree. Fortunately, thedegree can be surprisingly minimal, andthe quality of mylar filters within a par tic-ular brand tend to be uniform because ofconsistent manufacturing tolerances of thesubstrate and coatings.

With glass filters the accuracy ofthe optical surface (glass) must equal orexceed that of the other elements in theassembly or the performance of the tele-scope suffers; the greater the inaccuracies,the greater the distortion of the view.When seeing effects are taken into accountthese distortions are not so apparent at the

low magnifications used during whole discobserving. However, they become obviouswhen one attempts to increase magnifica-tion for close-up looks of solar detail.Granulation may never be visible, poresmay wash out and disappear, and finepenumbral filaments are beyond resolu-tion. Imagine viewing through a windowpane of your home. Some filters are madeof the same glass with a quality factor notmuch better.

Since optically flat plates are diffi-cult to manufacture the cost of proportion-ately larger sizes increases dramatically.Smaller than full aperture, but high qualityglass filters costing less can be used, but atthe price of reduced resolution (becausethe aperture of the telescope will now bedetermined by the sub-diameter of theobjective filter).

Many sources state that the opti-mum aperture for daytime solar observingis in the 4 to 5-inch range, and on mostdays this will be found to be tr ue. It isalso true, however, that occasionally localseeing conditions permit sub-arc secondviewing which can then be r ealized onlywith a larger aperture. Does the addedexpense of increased aperture justify thosefew times when sub-arc second viewing ispermitted? Amateurs such as Ar t Whipple(USA) and Wolfgang Lille (Germany) havesucessfully demonstrated the limited, butpowerful potential of larger than 5-inchaperture telescopes when used for whitelight solar observing.

So as you can clearly see, thechoice of a filter is determined b y a varietyof factors ranging from what features youwish to observe to the available funds. Arule of thumb in filter selection as withchoosing a telescope is to secure the finestquality product you can afford. It is point-less to obtain a poorly made filter that dis-torts the view through a high quality tele-scope and prohibits one from seeing as fineof detail as the telescope and atmospher epermit at a given time.



Testing a New Filter

When placing a new filter on thetelescope be certain that there is no possi-bility that the filter could come off unin-tentionally. Duct tape, and no astronomerworth his salt is without some, can be usedas a backup method of securing the filteronto the telescope.

After you get your filter(s) youmay want to perform a few simple testsbefore placing your eye in the way of theexit pupil. First, with the filter on the tele-scope and NOT pointed toward the Sun,but at a blank area of sky, take out the eye-piece. Now hold a white file card or sheetof paper up near the open end of thefocuser. You should not see light comingout of the focuser onto the car d. If you dothe filter is not safe for visual obser vationswithout either additional filtration or someform of further light rejection.

A second test, performed onlyafter the filtration is found safe, is to deter-mine the optical quality of your fil-ter/telescope combination. Wait for a daywhen the seeing is good. Use a highermagnification but not so high that theview becomes extremely dim. Find asunspot, a small umbral spot would be per-fect. Now slowly go from just inside focusto just outside. The spot should defocussymmetrically. If it becomes elongated inone direction on one side of focus andelongated perpendicular to that on theother side of focus, the filter should berejected. This is classicalastigmatism that can be caused by either apoor optical quality filter or a cell that isstraining or pinching the filter in somemanner. Be sure to also perform this teston a star (without the filter) at night firstto satisfy yourself that the problem is notthe telescope or collimation. I f it passesthis test look further for granulation, or amottled appearance to the sur face of thesun. The aperture of the telescope willhave to be four inches or gr eater, though

on occasion it has been photographed withsmaller. A six inch aper ture will readilyshow details in the penumbrae of sunspots.

A third check will determine thescatter in the filter/telescope combination.Scatter will decrease the contrast of solarfeatures, sometimes to the point of mask-ing detail. For this test you will not onlyneed pretty good seeing, but you will needa fairly dust and haze free day as well. Insky quality terminology this is known astransparency. If your site is never free ofthese, then pick a day when it is minimalgiven your prevailing conditions.

Astronomers frequently checktransparency by holding their hand at anarms length and blocking the Sun withtheir fist. For the best conditions thereshould be little or no bloom bey ond thefist. If the sky is still too bright to look atwith the Sun blocked completely by thefist then there is considerable scatteringinherent in the sky. As said above, this maybe unavoidable.

Using around 100x move the tele-scope so the solar limb cuts thr ough thecenter of the field. In the best of filters theregion beyond the limb will be black. Fewfilter/telescope/sky combinations will dis-play this. But if the sky beyond the limbappears extremely foggy or hazy and thelimb indistinct, even on the best days, itcould be a defect of the filter intr oducinglight scatter.



a ofexample

A collection of images from our observers.

Visible Features

As you look at the face of the S unfor the first time you may notice darkblotches on the visible sur face or photos-phere. These are sunspots. You will likelynotice too that the spots will cluster intobunches known as sunspot groups. Anincrease in magnification will reveal thatthe individual spots are themselves com-posed of several parts: a darker centercalled the umbra, surrounded by a lessdark or mottled penumbra. I f your sky isunusually calm and your filter a good one,you may well see that the penumbra iscomposed of dark hair-like penumbral fila-ments radiating outward from the umbrawith brighter penumbral grains trappedbetween them.

Individual sunspots may becrossed by bright streaks without internalstructure. These are light bridges. At thesame time you may notice the entire visiblesurface of the Sun is a mottled filigreeknown as granulation. Some of the cells ofthe granulation may be filled in but theywill not be as dark as the umbra insunspots. The larger of these are calledpores.

Surrounding the sunspot groups,seen especially well near the limb, arebright venous patches called faculae. Nowlet’s look at these and other featur es in alittle closer detail.

Limb Darkening

Jim Loudon, a space science lec-turer from the University of Michigan,gave this definition of a star, and the un isa star. He described it this way: “ The Sun,or any star for that matter, is a very big,very hot, ball of glowing gas.”

Because of gravity this gas is high-ly compressed toward the center andextremely hot. It gets cooler radially out-ward from the center and is coolest at thesurface or photosphere, about 5000K. The

proof of this radial decrease in temperatureis the limb darkening. Observing the solardisk projected onto a viewing screen youare looking through the photosphere at thehot, opaque center. But as you look moreand more toward the limb you are seeingless through the hot dense layers and morethrough the cooler less dense photosphericlayers. This gives the Sun a three dimen-sional spheroid appearance in the tele-scope, see figure 4.

Limb darkening is not the same atall wavelengths. It is stronger toward theultraviolet (UV) and almost nonexistent inthe infrared (IR). Further out in theextreme UV and shorter wavelengths thelimb tends to become brighter than thecenter of the disk. Here we are seeing thelayers above the photosphere where thetemperature rises again. By the time youbegin to approach the wavelengths of X-rays you are looking at the Sun's outeratmosphere, the solar corona.

Faculae

Faculae (plural of facula) are theextensive, bright filagree patches seen nearand often completely encompassingsunspot groups. Their contrast against thephotosphere is low but due to limb dark-ening they are well seen around the limb,as in figure 5. Occasionally a facular regionwill be bright enough to be seen w ell intothe disk but this is quite rar e. All sunspotgroups are associated with facular regionsbut the converse is not true. Faculae canexist without the presence of a sunspotgroup, as in the case of polar faculae,whereas sunspots have never been seen far-ther from the solar equator than the helio-graphic latitude of 50 degrees. Faculae usu-ally appear at a given location prior to theappearance of sunspots and often outliv e asunspot group by several rotations. Theveins of faculae exhibit a loose granulationwith a cell size of 1 to 2 ar c seconds.During the dissolution of a sunspot gr oup

White Light Features


Fig.4 Limb darkening

Fig.5 Faculae at the sol r limb

S

a

the facular material will form a light bridgethat quite often divides up one or mor e ofthe larger spots. It appears just to spreadand swamp the spot as the gr oup dissolves.

As stated above, faculae frequentlymark the site where a sunspot group justdissolved or is about to come into being.These faculae are usually big, bright andcompact. If the facular material is v erybright, this can mean that the sunspotgroup about to be born will be an activ eone. Observers should be alert to these fac-ulae. They should concentrate their observ-ing efforts on any newly formed bright fac-ulae or bright faculae that come ar oundthe limb, especially if there was not agroup at that longitude/latitude during theprevious rotation. These features give awarning of upcoming activity and indicateprevious activity that occurred on theopposing side of the un.

Light Bridges

These are loosely defined as anymaterial that is brighter than umbral mate-rial which divides an umbra, and oftenpenumbra as well. There seems to be arough correlation that the younger asunspot group is, the thinner will be thelight bridge structures that cross it. Theolder, more evolved groups commonlyhave light bridges that seem to be no mor ethan large incursions of photosphericmaterial. So, younger sunspot groups aremore likely to display the thin wispy butbright streamers through sunspots.

Both faculae and light bridgeshave a granular appearance. This may, atfirst, seem contradictory but this can beobserved using specialized techniques thatblock the bright light of the photospher e.Even the thin light bridges, the str eamers,in the best seeing resolve into small gran-ules. The lifetime of these light bridges canbe less than a day for the thinner ones to aweek or more for the more massive forms.The more massive bridges appearance usu-

ally signal the beginning of the end for asunspot group.

During the process of dissolution,the light bridge, which may star t out as abright facular like streamer, will graduallytake on the appearance of normal photos-pheric material. The process resembles theway a sinking ship is slowly swamped bythe ocean. A tell tale sign that this pr ocessis under way is a sunspot that is fan shapedwhere a light bridge has cut acr oss leavingpart of the umbral material dir ectly bor-dering the photosphere while the penum-bra is spread out like an oriental fan on theopposite side.

Granulation

The entire photosphere, or brightwhite light surface of the Sun, is dividedinto small convective cells, about 2-3 mil-lion over the whole Sun, called granula-tion. Each cell, known as a photosphericgranule, ranges in size from 1-5 arc sec-onds with most being 2-3 ar c seconds anda mean size being 2.5 arc seconds. Theyare separated from each other by thin bar-riers of darker material less than half an ar csecond in width called intergranular walls.The granulation and intergranular wallsenclosing the cells are evidence for convec-tion as a method of heat transfer in thisregion of the Sun. Like a bubbling pot ofoatmeal the heat is carried by the plasma asit moves upward, brightly hot, then stops,releases the energy and cools as it does,then falling back along the walls as dar kermaterial. Granules have short lifetime withmost surviving only 5 to 10 minutes.Generally, the longer the lifetime of a gran-ule the larger it will be. With telescopeapertures of 4 to 5-inches an obser ver maybegin to study granulation. However, theseeing at the observing site must be of thehighest quality. The observer must be verysensitive to changes in size, shape, andbrightness on a time scale as shor t as twominutes or less.



Fig.6 Light bridges

S

Pores

Among the photospheric granulesare some that are filled in by material thatis darker than the intergranular walls butnot as dark as the interior of an umbra.These are pores. They form and dissolve ina matter of a few minutes to an hour andrange in size from 1-5 arc seconds, withthe majority between 2-3 arc seconds. Likethe granulation there is a similar correla-tion between longevity and size. The largerpores have a good chance of sur viving andbecoming sunspots. Pores will tend to sur-vive longer and commonly exhibit slowerchanges than granules. Disturbancesbetween developing pores are common-place involving the formation of darkfilaments between them or simply changesin the granulation between them.

Sunspot Umbrae

As larger pores grow in size anddarkness they become increasingly stable.Some continue to grow in size and dark-ness (though the vast majority of pores donot) until they exceed the single pore sizelimit (about 5 arc seconds). These are nowumbral spots. Many of these will rapidlydevelop a rudimentary penumbra whichusually do not last long. The main distinc-tions of umbral spots from pores are thedarkness and size. They are usually irregu-lar in shape and, after acquiring a penum-bra, become quite ragged on theumbra/penumbra border. Additionally,penumbral filaments often appear tosprout from the umbra if the seeing isgood enough.

As alluded to above, the interiors



Fig.7 Big Bear Solar Observatory image showing various detail near a sunspot.

PORE

UMBRA

PENUMBRA

GRANULATION

PENUMBRALFILAMENTS

PENUMBRALGRAINS

of umbrae are not the smooth black thatthey first appear to be. They are composedof dark granular cells and small brightspots or points called umbral dots. Do notconfuse these with "umbral spots". Colorcan be noted within umbrae from a blackto a deep reddish-brown. In specializedlong exposure photography where all butthe light of the sunspot umbra is maskedoff, an umbral granulation similar indimensions to that of the photosphere isseen.

Umbral details require an aperturegreater than 4-inches as they ar e small.Umbral photographs where only the lightof the umbra is allowed to exit the tele-scope require exposures of only 2-5 timeslonger than photospheric exposures. Whatthis tells us is that the umbrae ar e only afew magnitudes dimmer than the sur faceof the Sun and would be brighter thanSirius if placed in the sky alone!

Sunspots are the result of surfacemagnetic fields disturbing the normal con-vection processes we spoke of earlier. Thesunspots are cooler and therefore appeardarker than the surrounding photospherebecause energy within the area of a sunspotthat is radiated away from the Sun cannotbe replaced as quickly through convectiondue to the sunspot's suppressingly greatermagnetic field.

Sunspot Penumbrae

Penumbrae often start out asintergranular material that is near to orbordering an umbral spot. As r udimentarypenumbrae they are usually dark and elon-gated and usually do not evolve past thisstage, especially if the umbral material isparticularly disorganized (i.e. only a collec-tion of small umbral spots scatter ed over afairly large area). These rudimentarypenumbrae are grouped into two classes:the evolving, or the dissolving. I f theumbra is large and well developed, thepenumbra will form a structure of dark

penumbral filaments in a radial systemabout the umbra. These filaments are simi-lar to the granulation in convective nature,but are modified by strong horizontal mag-netic fields. Between the dark, descendingfilaments are bright rising penumbralgrains. These should not be confused withthe granulation or photospheric granules.Overlying these filaments and grains ar ethe dark, shadowy, translucent fibrils. Atresolutions poorer than 1-arc second fibrilsand dark filaments cannot be separatelydistinguished. Mature, radially symmetricalpenumbrae are common about oldsunspots. But there is a more rare type ofpenumbra that is highly modified by com-plex magnetic fields. It is usually seenengulfing an entire sunspot group with fil-aments of different widths in differentareas of the sunspot group. The appearancecan be quite chaotic. At times islands ofthe penumbral material can be seendetached from any umbral material. This israre and observers should be alert to suchan appearance. This condition rarely lastsfor more than one day and a number ofobservations over a short time span shouldbe attempted to note any changes inappearance.

Within well developed penumbraecan be found dark islands of umbral mate-rial only slightly larger than pores.Sometimes the umbral material will str etchacross the penumbra and directly borderthe photosphere. Also, in the penumbrae,can be found bright regions of material asbright or brighter than the surroundingphotosphere. These dark and brightregions often go through rapid changesand should be watched closely. The brightregions may slowly be drawn out and fadeto become ordinary filaments or may growlarger and become a light bridge. I f one isvery fortunate and diligent, these brightintrusions may potentially be the begin-nings of a white light flar e!

The border between the umbraand penumbra is usually ragged with the



filaments looking like extensions of theumbra. A thin region may be seen hereappearing as a brightening of the penum-bra, this is termed the inner bright ring.The filaments are brightest near the umbraand get darker out towards the edge givingthe penumbra a well defined outer bound-ary. Beyond this where the filaments andfibrils project out into the photosphere,there can be seen several different phenom-enae. One is the brightening (about 10%)of the granules in a ring adjacent to thisouter boundary. This is termed the outerbright ring. The other is the organizationof the intergranular material in the adja-cent granules into rings concentric withthe outer boundary of the penumbra. Thegranules then appear to have formedchains about the penumbra. Often thegranules of these chains encroach on thepenumbra slowly dissolving it. Conversely,penumbrae are often created by the forma-tion of such chains about an umbral spotin which the intergranular material getsdarker and wider until the granules them-selves are compressed becoming the brightpenumbral grains.

To reiterate—a FIBRIL is a dark,semi-transparent, web-like structure thatoverlays the penumbra proper. Thepenumbra itself is composed of darkPENUMBRAL FILAMENTS and brightPENUMBRAL GRAINS, whereas thephotospheric granulation is composed ofgranules. These terms and several othersare sometimes misused and frequently con-fused.

Sunspot Evolution

As sunspots cross the visible diskof the un they exhibit many changes intheir apparent structure that have beenwell documented. When near the limbthere are several that are most obvious.Perhaps the most widely known of these isreferred to as the Wilson Effect.

When a rather symmetrical (circu-

lar) sunspot approaches the west limb ofthe Sun, the penumbra away from thelimb decreases in width more rapidly thanthe side closest to the limb. The reverse isseen for a spot coming into vie w on theeast limb. A sunken "saucer-like" impres-sion to the sunspot is the r esult.

This has been cited as proof thatsunspots are cavity like regions in the pho-tosphere. Recent measurements haveshown that the effect is not as pr ominentas once thought. Factors such as poor see-ing, photographic effects, photometricasymmetries of the foreshortened penum-bra, and the fact that sunspots ar e normal-ly more evolved on the west limb than theeast have all been used at one time oranother to account for the Wilson Effect.Astronomers today believe the depressioneffect is the result of gas within a spot'smagnetic field being thinner and less sub-stantial than the surrounding photosphere.The "thinner" gas of the sunspot is ther e-fore more transparent, allowing one tolook deeper into the photosphere. Sincethe magnetic field of the umbra is gr eater,one consequently sees further through thisregion than the penumbra.

To summarize, the evolution of anindividual sunspot goes as follows: For asmuch as a week or two before spot forma-tion a region of the Sun will exhibit thebright faculae. They form patchwork bestseen at the limb but with blue light (likethat passed by the Mylar-type filters) seenwell onto the disk. A granule will dar kenuntil its interior is as dark as the intergran-ular material thus becoming a pore. Mostpores do not evolve beyond this point butquickly fade away. Some however, willdarken and grow to three arc seconds orlarger until they acquire umbral intensity.These are then called umbral spots. Thevast majority of sunspots do not ev olvepast this stage. Next the formation of apenumbra takes place, rudimentary andirregular at first, but later it may becomesymmetrical and more extensive if the spot



S

evolves further. The penumbra may nowincrease and become more complex assmall motes of umbral material and brightspots appear within its boundaries. This isnow considered a full mature and activesunspot. It is usually surrounded by othersat this point and to continue this discus-sion further we must consider more com-plex structures.

Sunspot Groups

Sunspot Groups, simply put aremagnetically associated clusters ofsunspots. The sunspots within a groupmay be of differing ages and ar e often sur-rounded by numerous pores if the group isa well developed one. A group evolves inmore or less the following fashion.

A few pores will form a small clus-ter in an area of less than ten heliographicdegrees on the disk of the sun. After a dayor so these will darken and becomesunspots, often separating into two con-centrations. In each, one pore will developmore rapidly (in just a fe w hours) becom-ing a small sunspot. The leading concen-tration’s (“leading” in terms of the Sun’srotation) main spot will form more rapidlythan that of the following. Most groups donot evolve past this point and may staythis way for a few days and then dissolve.If the group continues to evolve, the leaderspot will usually form a penumbra fol-lowed shortly thereafter by penumbral for-mation about the small spots, and possiblysome detached penumbral material. Thesetwo sunspots will usually have pores,umbral spots, and detached penumbralmaterial between them. Now the largesunspots will rapidly separate in longitudewhile the axis of the group as measuredfrom the center of the large pr ecedingsunspot to the center of the follo wer, willrotate so its inclination with the solarequator decreases. These sunspot groupsare magnetically bipolar in that the leadingand following spots will be of an opposite

magnetic polarity. After these initial evolutionary

steps the group continues to grow in areaand number reaching a maximum sizearound the middle of the second w eek.Toward the end of that week or as late as amonth later the group will begin to breakup. First the smaller spots and pores willdissolve and then the following sunspotwill subdivide and shrink in siz e until ittoo is gone. All the while the leader or pr e-ceding spot is getting increasingly roundand symmetrical. Soon all that is left willbe the preceding spot, round with a uni-form penumbra about it, which simplyshrinks in size over several days or weeks.The faculae that preceded the formation ofthe group by many days will then be theonly remaining white light traces of activi-ty, living on for several additional weeks orlonger.

All this activity is the r esult ofmagnetic fields being generated by therotation of the Sun and the boilingmotions of convection in the Sun's outerlayers. Through these motions creation ofelectrical currents give birth to the magnet-ic fields, which then evolve through an allrepeating cycle of more current andstronger magnetism. Eventually, a field ispowerful enough to rise to the sur face withit's pocket of gas and become what isknown as an active region. This begins theevolutionary process spoken of earlier.

Sunspot and Sunspot GroupClassification

This scenario for the developmentof sunspots and sunspot groups has beendescribed by numerous systems of classifi-cation over the last several hundred years.The most popular and well know systemwas until recently, the one developed byM. Waldmeier of the Zurich Observatory,in 1938. Accordingly, this became knownas the Zurich Sunspot ClassificationSystem. The system consisted of nine class-



es lettered A through J, but omitting I. I tdelineated the characteristic evolutionarystages of sunspot groups, although not allgroups went through all classes duringdevelopment and decay. The majority ofgroups would go through only part of thesequence and then reverse their develop-ment or skip ahead to one of the laterclasses. Generally, the larger the area of thegroup, the more asymmetrical will be itsgrowth curve. While group areas tended toincrease from class A through J this wasnot absolutely the case. A large gr ouptends to have an asymmetrical growthcurve, rising rapidly from class A to E andthen decaying more slowly as it goes fromE to J, spending the majority of its decaytime in classes G to J.

Recently there has been a greateremphasis on the study of solar flar es, themost energetic events in our solar system.The Zurich Classification System workedpoorly as a predictor of which sunspotgroups would produce flares. It was knownthat groups of classes D, E, and F pr o-duced the most flares, however not all suchgroups produced flares. Even the mostactive and complex group, Class F, onlyhad a moderate chance of producing a flarein a given 24-hour period. A new classifi-cation system was devised by PatrickMcIntosh of the National Oceanic andAtmospheric Administration’s SpaceEnvironment Services Center in 1966. Hissystem is only slightly more complex butprovides a wealth of more informationabout individual sunspot groups. This sys-tem has been adopted by the ALPOSS inwhite light observing.

In the McIntosh system the classi-fication consists of three letters (i.e. Hsx).The first letter is a modified Z urich Class.This basically retains the old ZurichClasses but omits G and J which in thissystem would be redundant. This ModifiedZurich Class was used to be an induce-ment for seasoned observers that mightotherwise be reluctant to convert to the

new system. The second letter describesthe Largest Spot in the group; not neces-sarily the leading spot, but the largest one.The third letter assesses the SunspotDistribution within the group.

Let’s first define two critical terms:

Unipolar Group - A single spot, or com-pact cluster of spots, with the gr eatest sepa-ration between spots being less than threeheliographic degrees. With a Class Hgroup the separation is taken to be the dis-tance between the outer border of themain spot penumbra and the most distantattendant umbra.

Bipolar Group - Two or more spots form-ing an elongated cluster with a length ofthree or more heliographic degrees. If thereis a large principal spot, then the clustermust be greater than five degrees in extentin order to be bipolar.

Now we can go on to define the v ariousparameters of the system:

—MODIFIED ZURICH CLASS—

A - A unipolar group with no penumbra.This can be either the early or final stagein the evolution of a group.

B - A bipolar group with no penumbraeon any spots.

C - A bipolar group with penumbra onone end of the group, usually surroundingthe largest leader umbrae.

D - A bipolar group with penumbrae onspots at both ends of the gr oup and alength of less than 10 degrees.

E - A bipolar group with penumbrae onspots at both ends of the gr oup and a length of 10 to 15 degrees.



F - A bipolar group with penumbrae onspots at both ends of the group and alength greater than 15 degrees.

H - A unipolar group with a penumbra,usually the remains from a bipolar group.

—TYPE OF LARGEST SPOT—

x - No penumbra (groups of Class A or B).

r - Rudimentary penumbra that usuallyonly partially surrounds the largest spot.Penumbra will be granular rather than fila-mentary, appearing brighter than a maturepenumbra. The width of the penumbrawill be only a few granules, and may beeither forming or dissolving.

s - Small, symmetric spot with a mature,dark filamentary penumbra of circular orelliptical shape with a clean sharp border.If there are several umbrae in the penum-bra, they will form a tight cluster mimick-ing the symmetry of the penumbra. Thenorth-south diameter is 2.5 degrees or less.

a - Small, asymmetric spot with irregularsurrounding penumbra and with the con-tained umbrae separated from each other.The north-south diameter is 2.5 degrees orless.

h - A large symmetric spot; like type s, butthe north-south diameter is greater than2.5 degrees.

k - A large asymmetric spot; like type a,but the north-south diameter is greater than 2.5 degrees.

—SUNSPOT DISTRIBUTION—

x - Unipolar group of Modified ZurichClasses A or H (i.e. a solitary spot).

o - Open distribution with a leader and afollower spot and few or no spots betweenthem. Any spots between will be very smallumbral spots.

i - Intermediate distribution where numer-ous spots lie between the leader and fol-lower spots.

c - Compact distribution where the areabetween the leader and follower spots contains many spots with at least one hav-ing a penumbra. In extreme cases theentire group may be enveloped by onecomplex penumbra.

The illustration above is a graphicexample of how the classification systemworks. Remember that the first letter(upper case) is descriptive of the groupusing the Modified Zurich Class system;the second letter (lower case) refers to thelargest spot in the group; and the third let-ter (also lower case) is the assessment ofthe distribution of sunspots throughoutthe group. Practice in classifying sunspotgroups may be accomplished by comparing


20 © 2010 Association of Lunar & Planetary Observers - All rights reserved.

Eso

EaoDai Hsx

Figure 8. Panoramic view of several sunspot groupings from the 9th of July, 2000 by Art Whipple.The McIntosh Classification code is inserted above each group as an example of how the system functions.

visual appearances of spot groups at theeyepiece with the descriptions includedherein; then checking your work againstthe classifications logged daily by the pro-fessional observatories, many of which areposted on the internet. As the adage goes,practice makes perfect.

This is the system that allALPOSS observers who classify sunspotgroups are encouraged to use, especiallythose involved in the detection of solarflares. It only takes a minute or so longerto add the other two parameters but theinformation contained therein is muchgreater.

The Solar Cycle

The solar cycle was discovered byHeinrich Schwabe, a pharmacist inDessau, Germany. He was an amateurastronomer who used his observing to helphim escape the rigors of his inherited occu-pation as an apothecary. In the year 1826the project of observing and notingsunspots was suggested by a friend in anearby town. The paradigm of that timewas that sunspots were capricious and hap-hazard. The recording of them would seemno more than an idle pastime. Notedastronomers of that time and befor e werenearly unanimous in their declared belief

that there was no pattern or uniformity tothe appearance of sunspots. They weresimply a curiosity. Even the great Wm.Herschel had proclaimed that he “saw noreason to suspect that their abundance andscarcity were subject to orderly alterna-tion.” It is amazing that such a pr oject wassuggested, and even more so that it wasengaged! Schwabe was not under any illu-sions of great discoveries and undertookthe project in hopes of discovering anintermercurial planet. The plan was tosearch for a transit of such a body b y not-ing all black spots seen. Schwabe noted thecycle then made a preliminary announce-ment of his discovery of a ten year perio-dicity to sunspots. Subsequent observationsbore out his hypothesis which ev entuallybecame scientific law. In 1857 Schwabereceived the Royal Astronomical Society’sGold Medal for “his choice of an originaland appropriate line of work and in theadmirable tenacity of purpose with whichhe pursued it.” He observed the spots for atotal of 43 years and passed away on April11, 1875 at the age of 86.

Today the sunspot cycle is wellestablished by an additional 150 years ofaccurate observations and by further his-torical research. The number of sunspotsvaries with a period of 8 to 15 y ears withthe average being about 11.1 years. Note



Figure 10. Classic Butterfly Diagram of time plotted against latitude of sunspot dev elopment.Courtesy Hathaway/NASA/NSSTC

1920 1940 1960 1980

that this is half of a magnetic cy cle. Thislonger magnetic cycle was not discovereduntil the 20th century. Sunspot groups in agiven cycle in the north hemisphere mayhave leaders that are positive polarity andfollowers that are negative. During thatcycle the opposite will be tr ue in thesouthern hemisphere. In the next cycle thepolarities in each hemisphere will bereversed. Hence the magnetic polarity cyclein sunspots is two sunspot number cy cleslong. The mean maximum number ofsunspots can vary by as much as threetimes from cycle to cycle. The rise to maxi-mum is usually about three to four yearswith the remainder of the cycle being aslower decay to minimum. This rise israrely without irregularities and variancesin mean numbers of sunspots—as much as50% are common.

During the beginning of the cycle,at the low ebb, new cycle sunspot groupsform in high latitudes. These are recog-nized by the change in magnetic polarity.The alert observer may note the unusuallyhigh latitude (either north or south) andsuspect a new cycle spot. As the cycle pro-gresses the groups begin to appear in lower(more equatorial) latitudes. Until the endof a cycle the old cycle sunspot groups canbe seen hugging the heliographic equatorand the new cycle spots will be up around40-50 degrees latitude. This migration ofsunspot groups during a cycle was discov-ered and announced in 1863, by theEnglish amateur astronomer, RichardCarrington and later elaborated upon byGustav Sporer.

This variation of latitude with ageof the solar cycle prompted E.W. Maunder,well know for his discovery of a hiatus insolar cycles from about 1645-1700, to plotsunspot latitudes against time. The resultwas a diagram called the “ButterflyDiagram” (Fig.10) which clearly shows theequatorial migration of sunspot groupsduring the course of a solar cy cle. This dia-gram also shows that particularly active

cycles tend to have early spots at higherlatitudes. A plot of these for solar cy clesfrom the mid-19th century to presenthints at a periodicity in early sunspotsbeing formed at higher latitudes of a hun-dred years or more. This is in line withsuspected longer solar activity cycles andauroral records.

Recently, there has been sugges-tions that there may be some preference tosunspot group formation by longitudes.Observations bear this out on a shor t timescale of a few months to a year or more,but as yet longer term trends of this naturehave not been proven.

Solar Rotations

Because the Sun is not a solidbody it does not have a uniform rotationrate with respect to latitude. The spotsseen at the equator rotate faster (about 25days) than the spots nearer the poles(about 27 days at 30 degrees either side ofthe equator). The mean period is 25.38days but because of our own motion aboutthe sun, the synodic period is 27.2753days.

Solar rotations are based onRichard Carrington's photoheliographicseries from Greenwich in which rotationnumber one began on November 9, 1853.From that date the synodic period r otationnumbers, called Carrington Rotations aredetermined. Since that time there havebeen over 2000 rotations of the Sun.

Flares

Because of their importance toamateur and professional astronomers,flares are discussed in greater detail in aseparate section. Let it suffice her e to saythat flares are sudden releases of energyand mass from active regions. Most oftenthey are observed in the chromosphereusing Hydrogen Alpha or Calcium-K nar-row band filters on telescopes. I f they are



energetic enough, they can be seen atbroader band wavelengths and thusbecome White Light Flares (WLF). Thesewere once thought to be v ery rare eventsbut through some specialized techniques(not as expensive as the filtration men-tioned above) they can be observed onocassion in the more evolved groups.

In summary, there is a wide varietyof brightnesses among the solar phenome-na. The brightest are the white light flares,followed by faculae, normal photosphericmaterial within the granule walls, theintergranular material, pores, penumbrae,and lastly the dark sunspot umbrae.

Active Regions

An active region is as the nameimplies, a region of activity on the Sun.Active regions can contain one or moresunspots. The National Oceanic andAtmospheric Administration (NOAA)assigns consecutive numbers to activeregions as they are observed on the Sun,providing a way of cataloging this activity.Two observatories must observe a regionbefore it is assigned a number unless a flar eis observed within a region then it may benumbered before it is confirmed by thesecond observatory. The current number-ing system began on January 5, 1972 andhas been consecutive since that time. Atypical "name" for an active region may beAR6085, AR abbreviates active region. Asthe Sun rotates, the same active region maybe seen crossing the face of the Sun morethan once. If this is the case the r egion willbe given a new number, long lived activeregions may receive several numbers.

Active regions are referred to onlyby a 4-digit number, so when number10,000 was reached on June 14, 2002 allfollowing regions were still addressed bythe last four numbers of their name. F orexample, AR10056 is yet known asAR0056.

Observing Solar White Light Flaresby

Richard HillUniversity of Arizona

Lunar and Planetary Laboratory

Solar activity is not just gauged b y the number of sunspotsobserved. There are many other manifestations of solar activity also quan-tified that indicate well the level of activity. Flare production and strengthare two such parameters. Flares are sudden discharges of energy and sub-atomic particles that take place in and ar ound large sunspot groups asmagnetic fields change above the groups. Flares release prodigiousamounts of energy across most of the electromagnetic spectrum and arethus observable by a number of techniques. Larger flar es can emit asmuch as a thousandth the energy of the sun during the duration of thatflare. Subatomic particles are shot out at various speeds as well. Thesereleases take different times to traverse the space between Earth and Sunbut eventually impact the Earth's atmosphere causing changes in propa-gation of radio waves and the beautiful aurorae seen at temperate andpolar latitudes.

Typically, flares last a few minutes to as much as four hoursthough most are from ten to twenty minutes in duration. More energeticflares tend to be of longer duration, especially when obser ved in shorterwavelengths. In visible spectrum observations done by amateurs the rela-tionship is not quite as good. F lares are best seen in monochromatic lightsuch as H-alpha or the H and K lines of calcium wher e only light of oneabsorption line is allowed to enter the telescope. S ince flares are in emis-sion in these lines, whereas the rest of the disk of the S un is generally inabsorption, they appear quite bright against the disk. I n some cases flarescan be so energetic that they will ev en be seen in the light of the continu-um of the spectrum (between the dark absorption lines) as viewed in theamateur's telescope. These White Light Flares or WLFs were oncethought to be relatively rare.

Not all sunspot groups produce flares. In 1938, M. Waldmeierdevised the Zurich Sunspot Classification of these groups. It consists ofnine steps or classes (A through J, omitting I) that delineate characteristicevolutionary stages of sunspot groups, though not all groups go throughall classes. Most groups go only part way through the sequence and theneither rapidly go backwards through the classes or decay to the final class.In general, the greater the area of a group the more asymmetrical will beits growth curve. So a large group will rise rapidly from class A to E anddecay more slowly as it goes from E to J. Groups of classes D, E, and Fare the big flare producers. But not all such groups produce big flares.This was a problem for flare forecasters on whose work various broadcastand space industries depended. Even with the most active class, F, a fore-caster had a marginal chance of pr edicting flare probability in any given

White Light Flares


24 hour period. In order for flares to be studied, a r eliable system for identifying flare producing sunspot groups wasneeded. An observer would have to spend much time at the telescope obser ving every well developed group in hopes ofseeing these elusive events. It would have been highly advantageous, on the basis of a fe w parameters, to weed out manyless productive groups.

In 1966, Patrick McIntosh of the Space Environment Services Center of the National Oceanic and AtmosphericAdministration, introduced a sunspot classification system that impr oved the older Zurich system. The new classifica-tions consist of three letters. First is the Modified Zurich Class. It basically retains the old Zurich Class but G and J w ereremoved as being redundant. A Modified Zurich Class was used rather than a totally ne w system making it easier forobservers that might be reluctant to switch to the ne w system. The Second letter represents an assessment of the LargestSpot of the group. This is not necessarily the leading spot, but rather the L ARGEST. The third letter represents anassessment of the Spot Distribution within the group. It takes only slightly longer than the old system to classify all thegroups on the sun for a giv en day using the McIntosh System, but the information returned and usefulness of the ne wsystem makes it worth the slightly added effor t. In order to understand this system better, refer to pages 18-21 of thisbook for a discussion of Sunspot Group Classification using the McIntosh System.

This system has proven an accurate predictor of flares in the many years of its use. Indeed, it has helped solarastronomers understand better the relationship between flares and sunspots. Sunspot groups that produce flares are rela-tively rare. Because of this it has taken sev eral solar cycles of observations to demonstrate the effectiveness of the new sys-tem. Using the old Zurich system it was found that gr oups of class F were most likely to produce flares. But only a 40%flare probability in a 24 hour period could be pr edicted using this parameter alone. With the McIntosh System, usingModified Zurich Class F, the probability improved to 60%. Using just the Largest Spot class of "k" the probability in 24hours was 40-50%. If just Spot Distribution category "c" were used, flare probability went up to about 70%. But, whenall three dimensions of this system were used, classes Fsi, Fki and Fkc, sho wed a probability of up to 100% for pr oduc-tion of M flares in a 24 hour period and the M cIntosh Class of Fkc had a fur ther probability of up to 50% in X flar es(x-ray) production! This surpasses any former method of flar e prediction used, including sunspot area.

In optical regions of the spectrum flares are classified by size: (All degrees are heliographic.)

s - subflare of less than 2 degrees area.

1 - "Importance 1" flares, greater than 2 degrees but less than 5.1 degrees in area.

2 - "Importance 2" flares, greater than 5.1 degrees but less than 12.4 degrees area.

3 - "Importance 3" flares, greater than 12.4 degrees but less than 24.7 degrees in area.

4 - "Importance 4" flares, greater than 24.7 degrees in area.

and by their brightness:

F - faint or barely noticeable

N - Normal or noticeable

B - Bright or obvious

White Light Flares


This means that a 2B flar e is one that was bright and betw een 5.1 and 12.4 square heliographic degrees area. AnSF would be a faint subflar e, the most common type. For other parts of the electromagnetic spectrum, like x-ray, thereare other classification systems. But since amateur solar astronomers conducting a White Light Flare Patrol, or WoLFPatrol, will be observing in the visible spectrum these will not be discussed her e.

Flares occur in places where magnetic change is taking place or wher e the neutral line between areas of differentpolarity lies. There are some precursors to solar flares in these places. Filaments near the flare site may go into rapidmotion or may change brightness due to such motion in the line of sight if y ou are observing in relatively monochromat-ic light (doppler shifting). In larger flares the first sign is a pulse in har d x-ray and a slower pulsing in soft x-rays.Following this there may be flashes seen in longer wav elengths including optical. (This is again in rather monochromaticvisible light and I know of no case where this has been reported in broad band white light.) The pulses are caused byelectrons being shot through the corona at nearly half the speed of light causing oscillations in cor onal plasma above thesite which generates the radio bursts (called type III bursts) at fr equencies from about 10 to 800 Mhz (note its the FMBand). Receive these emissions and you will be forewarned of the optical flare.

Lacking a magnetograph, amateur astronomers must look for flares (in monochromatic light, continuum orbroad band white light) in the most common places wher e they occur. The priority list of McIntosh classes to bewatched are: Fkc, Fki, Ekc, Eki, Dkc, Dai, Dso and Hsx. These are the most flare productive groups of the 64 classes inorder of productivity. In the latter class, flares usually occur just beyond the outer penumbral boundary. PatrickMcIntosh once advised me to also watch gr oups that suddenly arrange their spots in a line. H e called this a "linearaccelerator" and a good bet as a site for flar es.

Within these groups one should watch:

-penumbrae that are chaotic, disturbed, or detached

-great clusters of smaller spots and penumbral bits betw een the main spots

-thin light bridges or light bridges caused b y detachment of penumbrae

-sunspots with or without penumbrae, that ar e breaking apart without reducing in area

-and rapidly moving spots in a group.

Observing WLFs requires some special equipment and precautions. The goal here is to maximize contrastbetween the flare and its surroundings. Thus all optics should be v ery clean since scattering from dust and other contam-inants on your optics will scatter light and r educe contrast. The telescope f/ratio should be long, f/20 or longer is goodand helps to reduce apparent defocussing from optical heating and normal, daytime seeing. A ccording to studies by Bray& Loughhead (1963), daytime seeing is 1 second of ar c only about 1% of the time at a good site. S o, reducing the aper-ture of your telescope, especially those big light buckets, to 4-6 inches will r esult in little or no loss of r esolution and willyield an improvement in image steadiness and contrast b y producing an unobstructed aperture. This will more thanmake up for any perceived losses.

You can increase your chance of seeing WLFs by increasing the contrast between the flare and bright photos-phere. This can be done through wideband filtration, unlike the narrow band filtration of only a fraction of an Angstr omused in H-alpha observations. A good region to filter around is at 4300 Angstroms or 430nm called the G-Band. Thereare a number of absorption lines cluster ed here that go into emission in flar es.When these normally dark lines becomebright the difference in brightness is greater than if you were looking at a bright r egion of the spectrum. With a narrowband filter the contrast is much gr eater. Since broad band filtration will take in a fair por tion of continuum it is still con-sidered "white light".

White Light Flares


Projection techniques will only detect the v ery brightest WLFs, explaining the paucity of them in the historicalrecords. We have never received a report of a WLF by anyone using the projection method. The most simple method ofobservation in searching for WLFs, is to just use a mylar-type filter . The blue image of these filters, normally detested b yamateurs, is quite close to our target wav elength. This is also the safest way to look for these flar es. These filters are alsovery good for showing faculae associated with these complex gr oups well in towards the centerof the disk. Only the highest quality filter should be used. There are some quality filters on the mar ket and some home-made ones out of lesser quality materials. These lesser quality show a good deal of sky brightness just off the limb of theSun. Such filters will probably scatter enough light to obliterate all but the brightest flar es.

Beyond this one can obtain filters of about 100 Angstr om bandpass to use at the ey epiece WITH A MYLAR-TYPE PREFILTER!! It may be necessary to reduce the density of the pr efilter but such experimentation should be donewith great caution. Do not risk your eyes at any time and NEVER USE EYEPIECE FILTRATION ALONE! A prefilteris a must unless you have a specially designed solar telescope. I f you are going to do this kind of obser ving you might dowell to build a specialized telescope for the task, but that will not be discussed her e. Some experimentation will likely benecessary if you want to go to narrower broad band filters but novices and the unsure are strongly encouraged to juststick with the commercial mylar-type filters. If you don't know, or aren't sure, don't do it!

When observing, first make a sketch of the r egion to be watched on the A.L.P.O. Solar Section Active Regionobserving form. Let the spot group fill the box. Once this is done observing may begin. Attempt to observe the region atleast once every ten to twenty minutes, the average lifetime of a flare. To check less often would risk missing one. B epatient! It may be quite a while befor e you bag the first one. I f you observe the McIntosh E and F groups cited in ourpriority list, you will be more likely to see one sooner. Do not sit constantly at the telescope. You will not see change. I tis too gradual and your eye needs the rest between observations.

Note ANY changes on the form. Not all flares behave the same way and not all pr ecursors are well known.Record time to the nearest second, start and stop. Surges have been photographed in white light but hav e only beenviewed at the limb (see Sky & Telescope, Dec., 1961, p.330). Some of the rapid changes r eported in penumbrae in his-tory may well have been observations of such surges. The only way to be sur e of such observations is to build a largerdata base of observations from which patterns may emerge.

Professional observations indicate that WLFs begin as a bright point of pr obably granule size in one of the sitesnoted earlier. My own observations during cycle 22 in AR5060 and 5062 ( June, 1988) tend to suppor t this position.Other points will pop up near the first in only a couple minutes, or the single one may be seen to enlarge rapidly . Theseearly stages are about all that happens in the smaller flar es, and in sub-flares the single point, substantially brighter thanthe photosphere, may be the full extent lasting only a fe w minutes. The human eye can detect only changes that ar e inexcess of about 10% against such a backgr ound so be aware of any brightening. Dr. Don Neidig, of Sacramento PeakSolar Observatory, in New Mexico, once expressed the suspicion that more such faint flares are visible in white light butbecause of the low contrast, short lifetime and smallness, go unnoticed.

In larger flares the points will grow in brightness, merge and become a bright ar ea or, if you are very lucky, abright ribbon. If you are so lucky, watch for a double ribbon (r unning on either side of the neutral line of magneticpolarity) or a shaded, penumbral-like ar ea near the flare which could be a surge.

Observing these, the most energetic ev ents in our solar system is an ex citing, and taxing business. It's taxing inthe long wait for the flar es and exciting in the activity when it happens. I t does not require long term commitment ofdaily observations but is a type of solar obser ving that can be done on the odd day when y ou have a couple hours tospare or while puttering about the yar d. You just go to the telescope ev ery ten minutes or so and make y our observation.At the risk of stating the ob vious, the observations will not make themselves. So make your observations and reportthem in a fashion where they can be useful. Thus, you will by pursuing your hobby, see the most energetic ev ents in oursolar system and be contributing to science. R emember, only a couple of hours on a S aturday afternoon may repay youwith a view of more energy than has been collectiv ely used by humans in all our y ears of existence!

White Light Flares


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

Association of Lunar & Planetary Observers …...Association of Lunar & Planetary Observers Association of Lunar & Planetary Observers Guidelines for the Observation of White Light

Documents