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 d Astrophysique de Paris CNRS Bis Boulevard Arago
F Paris France
Abstract We review the signi cant 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 white light
W L 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 con rmed the b ending of starlight
near the eclipsed Sun as predicted by Einstein s 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 co exist
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 di raction e ects 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
Zeeman s 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 non sunsp ot elds
Finally Bab co ck making use of a variety of new to ols photomultipliers
electro optic p olarization mo dulators narrow band ampli ers 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 o clo ck This
is a typical solar max corona in its complexity and its brightness Figure
shows a low lying 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 W W 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 de ection 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 con rmation 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 non LTE
Donald Menzel invented the jumping lm sp ectrograph to observe the ash
sp ectra of the chromosphere corona interface At the Khartoum eclipse of
February de nitive 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
d Astrophysique 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 di culties
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
Je eries Thomas for example that there was an increase of electron temp er
ature outward which required the development of non Lo 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 W L coronal images did not showany dramatic changes and pressure
W Livingston S Koutchmy
Figure Similar to Figure but showing north east structures and a
prominence The streamer on the left has a non radial edge suggesting
a sheet like 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 Sun s limb
and discover limb brightening
The Corona
Once the corona was historically identi ed as a part of the Sun new puzzles
arose Why didn t the corona scatter the Fraunhofer lines of the photosphere
What was the identi cation 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 e ect he deduced a kinetic temp erature of ab out
K
The eventual identi cation 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
at R
Eclipse Science Results Past and Present
Combine the ab ove information with the fact that the white light 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 Sun s 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 re ected by the lowchromosphere and do
not propagate further out in the chromosphere and so are ine ective 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 sur ng 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 b e inferred byseveral means but direct mea
surements would b e desirable Because the e ective Zeeman splitting is prop or
tional to wavelength there is a large advantage to making measurements in the
infrared An airb orne IR sp ectrometer was owby Olsen et at the eclipse
of March They observed the outer chromosphere and inner corona from
to and found emission lines Their sp ectra were re examined by Kaster
who up dated assignments based on improved data Judge has
predicted a number of forbidden transitions including that of Si IX at
Oliva et al have observed Si IX at in a Seyfert galaxy Kuhn et
al using a narrow band lter and imaging the corona at the February
eclipse mayhave detected emission from the Si IX line Lynchetal
prop ose to search for IR lines in the eclipse of June using sp ectrographs
designed to cover large areas of the corona The results could b e imp ortantfor
the design of the prop osed Advanced Solar Telescop e an instrument of p erhaps
meters ap erture
Fine Structure in the Corona
High resolution W L images p ermit us to visualize what is b elieved to b e current
sheets well inside the corona They are seen as tangential discontinuities TDs
W Livingston S Koutchmy
Figure Details in the inner corona from an observation at the
m CFHT on July This may be the highest resolution W L
coronal picture ever obtained
with a corresp onding jump in gas pressure Signatures of extended TDs have
been seen as a sharp edge to streamers imaged using a neutral radial lter
Koutchmy These TDs are seen over many radial scale heights which
means that a radial and rather laminar ow is resp onsible for the existence and
the stability of these structures They are also go o d candidates to b e the source
of the slow solar wind Additionally reconnections could o ccur inside the sheet
starting at a certain distance from the Sun as a result of instabilities and or
ts the in uence of the outer more global eld pro ducing plasmoids or fragmen
or detachments Koutchmy et al and releasing some energy Recent
SoHO observations made using the Lasco C W L coronagraph pro duced movies
showing the more external parts of the corona with very convincing evidence of
ows seen over the structures they were called blobs by the Lasco Observers
and their origin is certainly related to the discontinuities app earing in the low corona and seen during eclipses
Eclipse Science Results Past and Present
Figure Circumstances of the April BC eclipse that passed
over Babylon after Crump
In the inner corona the brightness is such that images can be taken with
very short exp osure times during eclipses This was done in at the fo cus of
the CFH telescop e in Hawaii Mauna Kea and the b est resolution W L images
were obtained see Figure Manyoverlapping lo ops and multiple sharp threads
are seen con rming that the magnetic eld is shaping all structures but that
dynamical e ects are also imp ortant This last p oint is certainly b etter studied
based on sp ectroscopic diagnostics which are also used at eclipses t
Time
Historic records of eclipses which extend back millenia can b e used to inves
tigate the rotation of the Earth Stephenson Crump Many ancient
records are questionable the exact date and even the place having been ad
Stephenson justed to t imp ortant events like battles or the death of kings
discusses this and concludes that the Babylonian eclipse of April BC is
probably rep orted correctly b ecause it is clearly describ ed on cuneiform tablets
in the British Museum without reference to any particular o ccasion Assuming
the Earth s rotation to b e constant and the same as to day acomputersimula
tion places the BC eclipse ab out km west of Babylon Figure Taking
into account tidal forces which slow the rotation the prediction is for km
east of Babylon Geophysicists prop ose a numb er of factors to account for this
error Like changes in the p olar ice cap from p ost glacial melting or sea level
changes asso ciated likewise with the evolution of glaciers
W Livingston S Koutchmy
Conclusions
It has been suggested that the Golden Age of eclipse science is over We have
presented some of the principle results from that era the concept of cosmic
magnetism the veri cation of the Einstein b ending of light a knowledge of
the physical state of the chromosphere and corona including the development
of non LTE diagnostics that are now commonplace for stellar and galactic sci
ence We foresee eclipse science continuing to prosp er however with increased
observations in the IR and the use of the new CCD detectors to perform high
resolution images and sophisticated sp ectroscopic diagnostics
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