THE SOLAR CORONA
"And what is the sun's corona?" mildly asked a college professor of a
student who might better have answered "Not prepared."
"I did know, Professor, but I have forgotten," was his reply.
"What an incalculable loss to science," returned the professor with a
twinkle. "The only man who ever knew what the sun's corona is, and he
has forgotten!"
Only in part has the mystery of the corona been cleared by the research
of the present day. Our knowledge proceeds but slowly, because the
corona has never been seen except during total eclipses of the sun; and
astronomers, as a matter of fact, have never had a fair chance at it.
Two total eclipses happen on the average of every three years; their
average duration is only two or three minutes; totality can be seen only
in a narrow path about a hundred miles wide, though it may be several
thousand miles long; there is usually about equal chance of cloud with
clear skies; and fully three-fourths of the totality areas of the globe
are unavailable because covered by water. So that even if we imagine the
tracks of eclipses quite thickly populated with astronomers and
telescopes, at least one every hundred miles, how much solid watching of
the corona would this permit? Only a little more than one week's time in
a whole century.
The true corona is at least a triple phenomenon and a very complex one.
The photographs reveal it much as the eye sees it, with all its
complexity of interlacing streamers projected into a flat, or plane,
surrounding the disk of the dark moon which hides the true sun
completely. But we must keep in mind the fact that the sun is a globe,
not a disk, and that the streamers of the corona radiate more or less
from all parts of the surface of the solar sphere, much as quills from a
porcupine.
From the sun's magnetic poles branch out the polar rays, nearly straight
throughout their visible extent. Gradually as the coronal rays originate
at points around the solar disk farther and farther removed from the
poles, they are more and more curved. Very probably they extend into the
equatorial regions, but it is not easy to trace them there because they
are projected upon and confused with the filaments having their origin
remote from the poles. Then there is the inner equatorial corona,
apparently connected intimately with truly solar phenomena, quite as the
polar rays are. The third element in the composite is the outer ecliptic
corona, for the most part made up of long streamers. This is most fully
developed at the time of the fewest spots on the sun. It is traceable
much farther against the black sky with the naked eye than by
photography. Without any doubt it is a solar appendage and possibly it
may merge into the zodiacal light.
Naturally this superb spectacle must have been an amazing sight to the
beholders of antiquity who were fortunate enough to see it. Historical
references are rare: perhaps the earliest was by Plutarch about A. D.
100, who wrote of it, "A radiance shone round the rim, and would not
suffer darkness to become deep and intense." Philostratus a century
later mentions the death of the emperor Domitian at Ephesus as
"announced" by a total eclipse.
Kepler thought the corona was evidence of a lunar atmosphere; indeed, it
was not until the middle of the 19th century that its lack of relation
to the moon was finally demonstrated. Later observers, Wyberd in 1652
and Ulloa, got the impression that the corona turned round the disk
catherine-wheel fashion, "like an ignited wheel in fireworks, turning on
its center." But no later observer has reported anything of the sort.
Quite the contrary, there it stands against the black sky in motionless
magnificence a colorless pearly mass of wisps and streamers for the most
part nebulous and ill-defined, fading out very irregularly into the
black sky beyond, but with a complex interlacing of filaments, sometimes
very sharply defined near the solar poles. It defies the skill of artist
and draughtsman to sketch it before it is gone.
Photograph it? Yes, but there are troubles. Of course the camera work is
superior to sketches by hand. As Langley used to say, "The camera has no
nerves, and what it sets down we may rely on." Foremost among the
photographic difficulties is the wide variation in intensity of the
coronal light in different regions of the corona. If a plate is exposed
long enough to get the outer corona, the exceeding brightness of the
inner corona overexposes and burns out that part of the plate or film.
If the exposure is short, we get certain regions of the inner corona
excellently, but the outer regions are a blank because they can be
caught only by a long exposure.
So the only way is to take a series of pictures with a wide range of
exposures, and then by careful and artistic handwork, combine them all
into a single drawing. Wesley of London has succeeded eminently in work
of this character, and his drawings of the sun's corona, visible at
total eclipses from 1871 onward, in possession of the Royal Astronomical
Society, are the finest in existence. They give a vastly better idea of
the corona, as the eye sees it, than any single photograph possibly can.
The early observers apparently never thought of the corona as being
connected with the sun. It was a halo merely, and so drawn. Its real
structure was neither known, depicted, or investigated. Sketches were
structureless, as any aureola formed by stray sunlight grazing the moon
might naturally be. That the rays are curved and far from radial round
the sun was shown for the first time in the sketches of 1842, and in
1860 Sir Francis Galton observed that the long arms or streamers "do not
radiate strictly from the center."
The inner corona had first been recorded photographically on a
daguerreotype plate during the eclipse of 1851, but the lens belonged to
a heliometer, and was of course uncorrected for the photographic rays.
The wet collodion plates of the eclipse of 1860, by De la Rue, proved
that not only the prominences but the corona were truly solar, because
his series of technically perfect pictures revealed the steady and
unchanged character of these phenomena while the moon's disk was passing
over them as totality progressed. And at the eclipse of 1869, Young put
the solar theory of the corona beyond the shadow of any further doubt
by examination of its light with the spectroscope and discovering a
green line in the spectrum due to incandescent vapor of a substance not
then identified with anything terrestrial, and therefore called
coronium.
The total brilliance of the corona was very differently estimated by the
earlier observers, though pretty carefully measured at later eclipses.
The standard full moon is used for reference, and at one eclipse the
corona falls short of, while at another it will exceed the full moon in
brightness. Variations in brilliancy are quite marked: at one eclipse it
was nearly four times as bright as the full moon. Much evidence has
already accumulated on this question; but whether the observed
variations are real, or due mainly to the varying relative sizes of sun
and moon at different eclipses, is not yet known. The coronal light is
largely bluish in tint, and this is the region of the spectrum most
powerfully absorbed by our atmosphere. Eclipses are observed by
different expeditions located at stations where the eclipsed sun stands
at very different altitudes above the horizon; besides this the
localities of observation are at varied elevations above sea level; so
that the varying amount of absorption of the coronal light renders the
problem one of much difficulty.
The long ecliptic streamers of the corona were first seen by Newcomb and
Langley during the totality of 1878. On one side of the sun there was a
stupendous extension of at least twelve solar diameters, or nearly 11
millions of miles. Langley observed from the summit of Pike's Peak, over
14,000 feet high, and was sure that he was witnessing a "real phenomenon
heretofore undescribed." The vast advantage of elevation was apparent
also from the fact that he held the corona for more than four minutes
after true totality had ended. These streamers are characteristic of the
epoch of minimum spots on the sun, as Ranyard first suggested. It was
found that this type of corona had been recorded also in 1867; and it
has reappeared in 1889, 1900 and 1911, and will doubtless be visible
again in 1922.
How rapidly the streamers of the corona vary is not known. Occasionally
an observer reports having seen the filaments vibrate rapidly as in the
aurora borealis, but this is not verified by others who saw the same
corona perfectly unmoving. Comparisons of photographs taken at widely
separate stations during the same eclipse have shown that at least the
corona remained stationary for hours at a time. Whether it may be
unchanged at the end of a day, or a week, or a month, is not known;
because no two total eclipses can ever happen nearer each other than
within an interval of 173 days, or one-half of the eclipse year. And
usually the interval between total eclipses is twice or three times this
period.
Theories of what the solar corona may be are very numerous. The extreme
inner corona is perhaps in part a sort of gaseous atmosphere of the sun,
due to matter ejected from the sun, and kept in motion by forces of
ejection, gravity, and repulsion of some sort. Meteoric matter is likely
concerned in it, and Huggins suggested the debris of disintegrating
comets. Schuster was in agreement with Huggins that the brighter
filaments of the corona might be due to electric discharges, but it
seems very unlikely that any single hypothesis can completely account
for the intricate tracery of so complex a phenomenon.
[Illustration: SOLAR CORONA AND PROMINENCES. Photographed during a
total eclipse of the sun, June 8, 1918. (_Courtesy, American
Museum of Natural History._)]
[Illustration: VENUS, SHOWING CRESCENT PHASE OF THE PLANET. Venus
is the earth's nearest neighbor on the side toward the sun.
(_Photo, Yerkes Observatory._)]
[Illustration: MARS, THE PLANET NEXT BEYOND THE EARTH. The
photograph shows one of the white polar caps. The caps are
thought to be snow or ice and may indicate the existence of
atmosphere. (_Photo, Yerkes Observatory._)]
Elaborate spectroscopic programs have been carried out at recent
eclipses, affording evidence that certain regions are due to
incandescent matter of lower temperature than the sun's surface. A small
part of the light of the corona is sunlight reflected from dark
particles possibly meteoric, but more likely dust particles or fog of
some sort. This accounts for the weakened solar spectrum with Fraunhofer
absorption lines, and this part of the light is polarized.
Many have been the attempts to see, or photograph, the corona without an
eclipse. None of them has, however, succeeded as yet. Huggins got very
promising results nearly forty years ago, and success was thought to
have been reached; but subsequent experiments on the Riffelberg in 1884
and later convinced him that his results related only to a spurious
corona. In 1887 the writer made an unsuccessful attempt to visualize the
corona from the summit of Fujiyama, and Hale tried both optical and
photographic methods on Pike's Peak in 1893 without success. He devised
later a promising method by which the heat of the corona in different
regions can be measured by the bolometer, and an outline corona
afterward sketched from these results.
Still another method of attacking the problem occurred to the writer in
1919, which has not yet been carried out. It would take advantage of
recent advances in aeronautics, and contemplates an artificial eclipse
in the upper air by means of a black spherical balloon. This would be
sent up to an altitude of perhaps 40,000 feet, where it would partake
of the motion of the air current in which it came to equilibrium. Then a
snapshot camera would be mounted on an aeroplane, in which the aviator
would ascend to such a height that the balloon just covered the sun, as
the moon does in a total eclipse. With the center of the balloon in line
with the sun's center, he would photograph the regions of the sky
immediately surrounding the sun, against which the corona is projected.
As the entire apparatus would be above more than an entire half of the
earth's atmosphere, the experiment would be well worth the attempt, as
pretty much everything else has been tried and found wanting. Needless
to say, the importance of seeing the corona at regular intervals
whenever desired, without waiting for eclipses of the sun, remains as
insistent as ever.