Eclipse Science Results: Past and Present
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The Last Total Solar Eclipse of the Mil lennium in Turkey ASP Conference Series Vol W Livingston and A Ozgu c eds Eclipse Science Results Past and Present W Livingston National Solar Observatory NOAO PO Box Tucson AZ USA S Koutchmy Institut dAstrophysique de ParisCNRS Bis Boulevard Arago F Paris France Abstract We review the signicant advances that havebeenachieved by eclipse exp eriments First we note the anomaly that the corona was not even seen until relatively mo dern times Beginning in whitelight WL photographs suggested structures that must have b een dominated by magnetic elds This led to the disco very of surface magnetism in sunsp ots by Hale in the rst detection of cosmic magnetism Deli cate direct photographic observations conrmed the b ending of starlight near the eclipsed Sun as predicted by Einsteins General Theory of Rela tivity Chromospheric ash sp ectra indicated that the outer layer of the Sun was much hotter than the underlying photosphere leading to non lo cal thermo dynamic equilibrium concepts and the idea of various me chanical and wave pro cesses to maintain it Decades have been devoted to understanding the corona Discovery of its million degree plus tem p erature and guring howto heat it how co ol prominences can coexist within it all are topics to which eclipse observations havecontributed Totality without a Corona Eclipses have b een chronicled for o ver years but the corona escap ed b eing noticed until mo dern times The single exception Leonis Deaconis in AD mentions a certain dull and feeble glow like a narrow headband shining round the extreme parts of the disc Stephenson Kepler made clear mention of it during the eclipse of Zirker but the rst real description came from Halley in who noted its structure and saw red prominences Kepler and Halley both thought it was lunar in origin Later it was thought that coronal rays are pro duced by diraction eects at the edge of the Mo on and many false drawings were made on this basis Why has the world p opulation been blind to what to day we consider one of the sp ectacular displays of nature Education for one thing we see what we have b een told is there It was also unlikely any single p erson would see more than one eclipse in a lifetime No one paid any attention to the corona since fear of the darkness and a concern ab out what the event p ortended probably dominated the exp erience The question arises what else are we missing W Livingston S Koutchmy Corona and Cosmic Magnetism In photography was intro duced as a way to record the corona Sites miles apart pro duced similar corona pictures This meantitwas not a terrestrial phenomenon Schwab e discovered the sunsp ot cycle in Subsequent eclipses from to revealed that the coronal structures varied with the activity cycle The corona was therefore solar in origin There was the strong suggestion that coronal structures mimicked the iron ling patterns of bar magnets Ab out this time Peter Zeeman discovered that sp ectrum lines split when formed in a magnetic eld George Ellery Hale inspired by eclipse photographs and Zeemans nding lo oked for and found magnetic elds in sunsp ots This was the rst concrete detection of cosmic magnetism However eclipse pictures also indicated a complex of surface magnetism outside sunsp ots The MountWilson observers struggled for decades in vain to detect these weak nonsunsp ot elds Finally Bab co ck making use of a variety of new to ols photomultipliers electrooptic p olarization mo dulators narrowband ampliers develop ed the solar magnetograph Extrap olated surface elds now could map out and to a extent explain coronal structure The corona on August Figure certainly is dominated by magnetic structures We see helmuts rays and a very nice lo op at ab out oclo ck This is a typical solar max corona in its complexity and its brightness Figure shows a lowlying prominence whichwas pinkish in the original color recording We can presume this color arises from the contribution of b oth H red and H blue emission Einstein Light Bending It is a valuable theory that predicts fundamentally new phenomena Einstein develop ed his General Theory of Relativity one consequence was that a lightray would deviate arcsec toward the Sun at R Recall that thetheorywas not widely accepted around WW Camb ell Director of Lick Observatory for example said he hop ed it would not be true Two eclipse exp editions in carried out light deection exp eriments Star elds around the eclipsed Sun were photographed and the pro cess rep eated six months later with the same instrument and due attention to thermal and emulsion stability The results were announced with great fanfare bythe Presidentof the Royal So ciety and arcsec a conrmation Light deviation measurements were rep eated at eclipses from to all with p ositive results Radio astronomers had the nal word when F omalont and Sramek used the km baseline interferometer at Greenbank to obtain a deviation of arcsec Chromosphere and nonLTE Donald Menzel invented the jumping lm sp ectrograph to observe the ash sp ectra of the chromospherecorona interface At the Khartoum eclipse of February denitive observations were obtained by Bernard Lyot and by JackEvans the former died in Egypt after the exp edition and the latter broke Eclipse Science Results Past and Present Figure Image of the corona on August obtained by Institut dAstrophysique Paris in Chadegan Iran A radial optical lter plus image pro cessing have equalized the inner and outer parts a leg The Lyot sp ectra were nally brought to France with great diculties their pro cessing revealed several new coronal lines The b ehavior of the emission lines without a continuum indicated to later analysts Pecker Michard Athay Jeeries Thomas for example that there was an increase of electron temp er ature outward which required the development of nonLo cal Thermo dynamic Equilibrium metho ds Hiei and Hirayama obtained sp ectra to the heights of km See the new book Total Eclipses by Guillermier and Koutchmy for something of the history of this exciting era Besides the ground based eclipse exp eriments a new breed of observers to ok to high ying aircraft Not only did they get ab ovemuc h of the water absorption but they could extend the time of totality bychasing the shadow A Concorde ight on June yielded minutes of totality The movie made from the numerous WL coronal images did not showany dramatic changes and pressure W Livingston S Koutchmy Figure Similar to Figure but showing northeast structures and a prominence The streamer on the left has a nonradial edge suggesting a sheetlike distributionn on the plasm density waves as exp ected However some p olar plumes did show some subtle variations suggesting waves are propagating inside Probably the sho ckwaves surrounding the sup ersonic aircraft was a source of image distortion and bad seeing Other results concerned IR sp ectra of the outer corona but these measurements were rather noisy A NASA Kuip er Airb orne Observatory ight allowed Lindsey and team to explore and radiation at the Suns limb and discover limb brightening The Corona Once the corona was historically identied as a part of the Sun new puzzles arose Why didnt the corona scatter the Fraunhofer lines of the photosphere What was the identication of the strong green and red emission lines found there Actually Grotrian did nd the strongest Fraunhofer line Ca K in the corona This line was noticeably broader than in the photosphere and assuming it was a thermal eect he deduced a kinetic temp erature of ab out K The eventual identication by Edlen of Fe XIV A and Fe X A as forbidden lines that require an ionization temp erature of K for their p opulation is a triumph of eclipse science The width of these lines is also compatible with this order of temp erature Kim The widths of Lyman as observ ed from space showa run of temp erature from at R to atR Eclipse Science Results Past and Present Combine the ab ove information with the fact that the whitelight or K corona is p olarized and we have that the corona is a primarily a tenuous cloud of million degree electrons The light we see is due to Thompson scattering of light from the photosphere The corona b eing electrically conducting magnetic elds with their fo otp oints on the solar surface are the source of its varied forms helmet streamers rays brushlike tufts near the p oles at solar maximum asortofgiant dahlia What Heats the Corona The degree corona cannot b e heated by conduction from the photosphere or chromosphere as this violates the nd law of thermo dynamics Only of the Suns radiative output energy is required however Biermann pro p osed acoustic heating from granular convection This promising idea had to b e abandoned b ecause acoustic waves are reected by the lowchromosphere and do not propagate further out in the chromosphere and so are ineective they are standing waves The consensus now points to the role of magnetic elds say by the dissipation of propagating MHD waves via ohmic heating Narain and Ulmschneider Golub P asacho Recently the Ultraviolet Coro nal Sp ectrometer ab oard SOHO has detected such vibrating waves accelerating particles by a pro cess analogous to surng Crammer et al Another con cept p ossibly more controversial but easily understo o d is an idea b y Scudder wherein the high velo city tail of particle velo city distribution gives rise to sho cks in the tenuous corona Infrared Sp ectral Diagnostics Magnetic elds in the corona can