SUN SPOTS AND PROMINENCES
Dark spots of a deep bluish black will often be seen on the photosphere
of the sun. Sometimes single, though generally in groups, the larger
ones will have a dark center, called the umbra, surrounded by the very
irregular penumbra which is darker near its outer edge and much brighter
apparently on its inner edge where it joins on the umbra. The penumbra
often shows a species of thatch-work structure, and systematic sketches
of sun spots by observers skilled in drawing are greatly to be desired,
because photography has not yet reached the stage where it is possible
to compete with visual observation in the matter of fine detail. The
spots themselves nearly always appear like depressions in the
photosphere, and on repeated occasions they have been seen as actual
notches when on the edge of the sun.
Many spots, however, are not depressions: some appear to be actual
elevations, with the umbra perhaps a central depression, like the crater
in the general elevation of a volcano. Spots are sometimes of enormous
size. The largest on record was seen in 1858; it was nearly 150,000
miles in breadth, and covered a considerable proportion of the whole
visible hemisphere of the sun. A spot must be nearly 30,000 miles across
in order to be seen with the naked eye.
In their beginning, development, and end, each spot or group of spots
appears to be a law unto itself. Sometimes in a few hours they will
form, though generally it is a question of days and even weeks. Very
soon after their formation is complete, tonguelike encroachments of the
penumbra appear to force their way across the umbra, and this splitting
up of the central spot usually goes on quite rapidly. Sun spots in
violent disturbance are rarely observed. As the sun turns round on his
axis, the spots will often be carried across the disk from the center to
the edge, when they become very much foreshortened. The sun's period of
rotation is 28 days, so that if a spot lasts more than two weeks without
breaking up, it may reappear on the eastern limb of the sun after having
disappeared at the western edge. Two or three months is an average
duration for a spot; the longest on record lasted through 18 months in
1840-41.
The position of the sun's axis is well known, its equator being tilted
about 7 degrees to the ecliptic, and the spots are distributed in zones
north and south of the equator, extending as far as 30 degrees of solar
latitude. In very high latitudes spots are never seen; they are most
abundant in about latitude 15 degrees both north and south, and rather
more numerous in the northern than in the southern hemisphere of the
sun. Recent research at Mount Wilson makes the sun a great magnet; and
its magnetic axis is inclined at an angle of 6 degrees to the axis of
rotation, around which it revolves in 32 days.
There is a most interesting periodicity of the spots on the sun, for
months will sometimes elapse with spots in abundance and visible every
day, while at other periods, days and even weeks will elapse without a
single spot being seen. There is a well recognized period of eleven and
one-tenth years, the reason underlying which is not, however, known.
After passing through the minimum of spottedness, they begin to break
out again first in latitudes of 25 degrees-30 degrees, rather suddenly,
and on both sides of the equator, and they move toward the equator as
their number and individual size decrease.
The last observed epoch of maximum spot activity on the sun was passed
in 1917.
Many attempts have been made to ascertain the cause of the periodicity
of sun spots, but the real cause is not yet known. If the spots are
eruptional in character, the forces held in check during seasons of few
spots may well break out in period. The brighter streaks and mottlings
known as faculae are probably elevations above the general photosphere,
and seem to be crusts of luminous matter, often incandescent calcium,
protruding through from the lower levels. Generally the faculae are
numerous around the dark spots, and absorption of the sun's light by his
own atmosphere affords a darker background for them, with better
visibility nearer the rim of the solar disk. The spectroheliograph
reveals vast zones of faculae otherwise invisible, related to the
sun-spot zones proper on both sides of the equator.
In some intimate way the magnetism of sun and earth are so related that
outbreaks of solar spots are accompanied with disturbances of electrical
and other instruments on the earth; also the aurora borealis is seen
with greater frequency during periods when many spots are visible.
Within very recent years the discovery of a magnetic field in sun spots
has been made by Hale with powerful instruments of his own design. Sun
spots had never been investigated before with adequate instrumental
means. He recognized the necessity of having a spectroscope that would
record the widened lines of sun-spot spectra, and the strengthened and
weakened lines on a large scale. Certain changes in relative intensity
were traced to a reduced temperature of the spot vapors by comparison
with photographs of the spectrum of iron and other metallic vapors in an
electric arc at different temperatures. Here the work of the laboratory
was essential. Sun spots were thus found to be regions of reduced
temperature in the solar atmosphere. Chemical unions were thus possible,
and thousands of faint lines in spot-spectra were measured and
identified as band lines due to chemical compounds. Thus the chemical
changes at work in sun-spot vapors were recognized.
Then followed the highly significant investigations of solar vortices
and magnetic fields. Improvements in photographic methods had revealed
immense vortices surrounding sun spots in the higher part of the
hydrogen atmosphere; and this led to the hypothesis that a sun spot is a
solar storm, resembling a terrestrial tornado, and in which the hot
vapors whirling at high velocity are cooled by expansion. This would
account for the observed intensity changes of the spectrum lines and the
presence of chemical compounds. The vortex hypothesis suggested an
explanation of the widening of many spot lines, and the doubling or
trebling of some of them. As it is known that electrons are emitted by
hot bodies, they must be present in vast numbers in the sun; and
positive or negative electrons, if caught and whirled in a vortex, would
produce a magnetic field.
Zeeman in 1896 had discovered that the lines in the spectrum of a
luminous vapor in a magnetic field are widened, or even split into
several components if the field is strong enough. Characteristic effects
of polarization appear also. The new apparatus of the observatory in
conjunction with experiments in the laboratory immediately provided
evidence that proved the existence of magnetic fields in sun spots, and
strengthened the view that the spots are caused by electric vortices.
Extended investigations have led Hale to the conclusion that the sun
itself is a magnet, with its poles situated at or near the poles of
rotation. In this respect the sun resembles the earth, which has long
been known to be a magnet. The sun's axial rotation permits
investigation of the magnetic phenomena of all parts of its surface, so
that ultimately the exact position of the sun's magnetic poles and the
intensity of the field at different levels in the solar atmosphere will
be ascertained. Schuster is of the opinion that not only the sun and
earth, but every star, and perhaps every rotating body, becomes a magnet
by virtue of its rotation. Hale is confident that the 100-inch reflector
will permit the test for magnetism to be applied to a few of the stars.
The sun can be observed at Mount Wilson on at least nine-tenths of all
the days in the year, and a daily record of the polarities of all spots
with the 150-foot tower telescope is a part of the routine. A method has
been devised for classifying sun spots on the basis of their magnetic
properties, and more than a thousand spots have already been so
classified. About 60 per cent of all sun spots are found to be binary
groups, the single or multiple members of which are of opposite magnetic
polarity. Unipolar spots are very seldom observed without some
indication of the characteristics of bipolar groups. These are usually
exhibited in the form of flocculi following the spot. The bipolar spot
seems to be the dominant type, and the unipolar type a variant of it.
Although devised for quite another purpose, that of photographing the
hydrogen prominences on the limb of the sun, the spectroheliograph has
contributed very effectively to many departments of solar research. The
prominences are dull reddish cloudlets that were first seen during total
eclipses of the sun. Probably Vassenius, a Swedish astronomer, during
the total eclipse of 1733, made the earliest record of them, as pinkish
clouds quite detached from the edge of the moon; and in that day, when
it had not yet been proved that the moon was without atmosphere, he
naturally thought they belonged to the moon, not the sun. Undoubtedly
Ulloa, a Spanish admiral, also saw the prominences in observing the
total eclipse of 1778; but they seem to have attracted little attention
till 1842, when a very important total eclipse was central throughout
Europe, and observed with great care by many of the eminent astronomers
of all countries.
So different did the prominences appear to different eyes, and so many
were the theories as to what they were, that no general consensus of
opinion was reached, and some thought them no part of either sun or
moon, but a mere mirage or optical illusion. But at the return of this
eclipse in 1860, photography was employed so as to demonstrate beyond a
shadow of doubt the real existence and true solar character of the
prominences. By the slow progress of the moon across the sun and the
prominences on the edge, a unique series of photographs by De la Rue
showed the moon's edge gradually cutting off the prominences piecemeal
on one side of the sun, and equally gradually uncovering them on the
opposite side.
The prominences, then, were known to be real phenomena of the sun, some
of them disconnectedly floating in his atmosphere, as if clouds. Their
forms did not vary rapidly, they were very abundant, and their light was
so rich in rays of great photographic intensity that many were caught on
the plate which the eye failed to see; they appeared at every part of
the sun's limb and their height above it indicated that they must be
many thousand miles in actual dimension. What they were, however,
remained an entire mystery, and no one even thought it possible to find
out what their chemical constitution might be or to measure the speed
with which they moved.
A few years later came the great Indian eclipse (August 28, 1868), at
that date the longest total eclipse ever observed. Janssen of France and
many others went out to India to witness it. Fortunately the prominences
were very brilliant and this led Janssen to believe it would be possible
for him to see them the day after the eclipse was over. By modifying the
adjustment of his apparatus suitably and changing its relation to the
sun's edge, he found that hydrogen is the main constituent in the light
of the prominences. In addition to this he was able to trace out the
shapes of the prominences, and even measure their dimensions. His
station in India was at Guntoor, many weeks by post from home; so that
his account of this important discovery reached the Paris Academy of
Sciences for communication with another from the late Sir Norman Lockyer
of England, announcing a like discovery, wholly independently.
The principle is simply this, and admirably stated by Young: "Under
ordinary circumstances the prominences are invisible, for the same
reason as the stars in the daytime: they are hidden by the intense light
reflected from the particles of our own atmosphere near the sun's place
in the sky; and if we could only sufficiently weaken this aerial
illumination, without at the same time weakening _their_ light, the end
would be gained. And the spectroscope accomplishes this very thing.
Since the air-light is reflected sunshine, it of course presents the
same spectrum as sunlight, a continuous band of color crossed by dark
lines. Now, this sort of spectrum is greatly weakened by every increase
of dispersive power, because the light is spread out into a longer
ribbon and made to cover a more extended area. On the other hand, a
spectrum of bright lines undergoes no such weakening by an increase in
the dispersive power of the spectroscope. The bright lines are only more
widely separated--not in the least diffused or shorn of their
brightness."
Simultaneous announcement of this great discovery, by astronomers of
different nations, working in widely separate regions of the earth, led
to the striking of a gold medal by the French Government in honor of
both astronomers and bearing their united effigies. Ever since the
famous Indian eclipse of 1868, it has not been necessary to wait for a
total eclipse in order to observe the solar prominences, but every
observer provided with suitable apparatus has been able to observe them
in full sunlight whenever desired, and the charting of them is part of
the daily routine at several observatories in different parts of the
world. So vast has been the accumulation of data about them that we know
their numbers to fluctuate with the spots on the sun; and their
distribution over the sun's surface resembles in a way that of the
spots.
While the spots and protuberances are most numerous around solar
latitude 20 degrees both north and south, the prominences do not
disappear above latitude 35 to 40 degrees, as the spots do, but from
latitude 60 degrees they increase in number to about 75 degrees, and are
occasionally observed even at the sun's poles. Faculae and prominences
are more closely related than the sun spots and prominences. There are
wide variations in both magnitude and type of the prominences. Heights
above the sun's limb of a few thousand miles are very common, and they
rarely reach elevations as great as 100,000 miles, though a very
occasional one reaches even greater heights.
Classification of the prominences divides them into two broad types, the
quiescent and the eruptive. The former are for the most part hydrogen,
and the latter metallic. The quiescent prominences resemble closely the
stratus and cirrus type of terrestrial clouds, and are frequently of
enormous extent along the sun's edge. They are relatively long-lived,
persisting sometimes for days without much change. The eruptive
prominences are more brilliant, changing their form and brightness
rapidly. Often they appear as brilliant spikes or jets, reaching
altitudes that average about 25,000 miles. Rarely seen near the sun's
poles, they are much more numerous nearer the sun spots. Speed of
motion of their filaments sometimes exceeds one hundred miles a second,
and the changing variety of shapes of the eruptive prominences is most
interesting. Oftentimes they change so rapidly that only photography can
do them justice.
Prominence photography began with Young a half century ago, who obtained
the first successful impression on a microscope slide with a sensitized
film of collodion; as was necessary in the earlier wet-plate process of
photography, which required exposures so long that little progress was
effected for about twenty years. Then it was taken up by Deslandres of
Paris and Hale of Chicago independently, both of whom succeeded in
devising a complex type of apparatus known as the spectroheliograph, by
which all the prominences surrounding the entire limb of the sun can be
photographed at any time by light of a single wave-length, together with
the disk of the sun on the same negative.
The prominences appear to be intimately connected with a gaseous
envelope surrounding the solar photosphere, in which sodium and
magnesium are present as well as hydrogen. The depth of the chromosphere
is usually between 5,000 and 10,000 miles, and its existence was first
made out during the total solar eclipses of 1605 and 1706, when it
appeared as an irregular rose-tinted fringe, though not at the time
recognized as belonging to the sun.
The constitution of the sun and its envelopes are still under
discussion, and no complete theory of the sun has yet been advanced
which commands the widest acceptance. Of the interior of the sun we can
only surmise that it is composed of gases which, because of intense heat
and compression, are in a state unfamiliar on earth and impossible to
reproduce in our laboratories. Their consistency may be that of melted
pitch or tar.
Surrounding the main body of the sun are a series of layers, shells, or
atmospheres. Outside of all and very irregular in structure, indeed
probably not a solar atmosphere at all, is the solar corona, parts of
which behave much as if it were an atmosphere, but it appears to be
bound up in some way with the sun's radiation. It has streamers that
vary with the sun-spot period, but its constitution and function are
very imperfectly known, because it has never been seen or photographed
except at rare intervals on occasion of total eclipses of the sun.
Beneath the corona we meet the projecting prominences, to which parts of
the corona are certainly related, and beneath them the first true layer
or atmosphere of the sun known as the chromosphere, its average depth
being about one-hundredth part of the sun's diameter. Beneath the
chromosphere is the layer of the sun from which emanates the light by
which we see it, called the photosphere. It appears to be composed of
filaments due to the condensation of metallic vapors, and it is the
outer extremities of these filaments which are seen as the granular
structures everywhere covering the disk of the sun. Their light shines
through the chromosphere and the spots are ruptures in this envelope.
Between photosphere and chromosphere is a very thin envelope, probably
not over 700 miles in thickness, called the reversing layer. It is this
relatively thin shell that is responsible for the absorption which
produces the dark lines in the spectrum of the sun. Under normal
conditions the filaments of the photosphere are radial, that is vertical
on the sun; but whenever eruptions take place, as during the occurrence
of spots, the adjacent filaments are violently swept out of their normal
vertical lines and these displaced columns then form what we view as the
spot's penumbra. From the outer surface of the sun's chromosphere rise
in eruptive columns vapors of hydrogen and the various metals of which
the sun is composed. These and the spots would naturally occur in
periods just as we see them.
We have said that the sun is composed of a mass of highly heated or
incandescent vapors or gases, whose compression on account of gravity
must render their physical condition quite different from any gaseous
forms known on the earth or which we can reproduce here. As the result
of more than half a century of studious observation of the sun and
mapping of its spectrum in every part, and diligent comparison with the
spectra of all known chemical elements on the earth, we find that the
sun contains no elements not already found here, but that a great
preponderance of elements known to earth are found in the sun.
The intensity of their spectral lines is one prominent indication of the
presence of elements in the sun, and the number of coincidences of
spectral lines is another. Iron, nickel, calcium, manganese, sodium,
cobalt, and carbon are among the elements most strongly identified. A
few of the rarer terrestrial elements are of doubtful existence in the
sun, and a very few, as gold, bismuth, antimony, and sulphur are not
found there, and the existence of oxygen in the sun is regarded by some
experts as doubtful. But if the whole earth were vaporized by heat,
probably its spectrum would resemble that of the sun very closely.
What are the effects of the sun, and sun spots in particular, on our
weather? Is the influence of their periodicity potent or negligible? If
we investigate conditions pertaining to terrestrial magnetism, as
fluctuations of the magnetic needle, and the frequency of aurorae, there
is no occasion for doubt of the sun's direct influence, although we are
not able to say just how that influence becomes potent. If, however, we
look into questions of temperature, barometric pressure, rainfall,
cyclones, crops, and consequent financial conditions, we find fully as
much evidence against solar influence as for it. The slight variations
of the sun's light and heat due to the presence or absence of sun spots
can scarcely be sensible, and much longer periods of closer observation
are necessary before such questions can be finally decided. The slighter
such influences are, if they actually exist, and the more veiled they
are by other influences more or less powerful, the more difficult it is
to discover their effects with certainty.
The importance of solar radiation in the prediction of terrestrial
weather has long been recognized, but until very recently no practical
application has been made. The Smithsonian Astrophysical Observatory at
Washington, under the direction of Dr. Abbot, has for many years carried
on at a number of stations a series of determinations of the constant of
solar radiation by the spectro-bolometric method originated by Langley.
A new station in Calama, Chile, has recently been inaugurated, at which
the solar constant is worked out each day, and telegraphed to the
Argentine weather service, where it is employed in forecasting for the
day.
Abbot's new method of solar constant determination is based on the fact
that atmospheric transparency varies oppositely to the variations of
brightness of the sky. Increase of haziness presents more reflecting
surface to scatter the solar rays indirectly to the earth. Of course it
presents also additional surface to obstruct the direct rays from the
sun. By measuring the brightness of the sky near the sun, it becomes
possible to infer the coefficients of atmospheric transmission at all
wave lengths. The direct observations and the complete deduction of the
solar constant for the day can all be completed within two or three
hours.
Clayton of Buenos Aires has now employed these results in the Argentine
weather predictions for two years, and the introduction of this new
element in forecasting has brought about a pronounced gain in the value
of the predictions. Its adoption by the weather bureaus of other nations
will doubtless come in due time, and the new method take a firmly
established rank in practical meteorology.
Abbot's observations many years ago first called attention to the
variability of the solar constant through a range of several per cent
both from year to year, and in irregular short periods of weeks or even
days. Abbot considers this the more likely explanation than that
atmospheric changes should take place simultaneously all over the earth.
The sun is but a star, the stars that are irregularly variable in light
and heat are numerous, and the sun itself appears to be one of these.
Especially important to the agricultural and vineyard interests of
Argentina is the question of precipitation, and Clayton finds this very
dependent on solar radiation. At epochs of practically stationary solar
intensity, there is little or no precipitation; but quite generally he
finds that great decrease of solar radiation is followed in from three
to five days by heavy precipitation. Direct temperature effects are also
traced in Buenos Aires and other South American cities, lagging from two
to three days behind the observed solar fluctuations.
The station at Calama yields about 250 determinations of the solar
constant each year, and the Mount Wilson station about half that number.
They are the only stations of this character at present in existence,
and others should be established in widely separated and cloudless
regions, as Egypt, southern California and Australia. Uniformity in the
methods of observing would be highly desirable, and the Smithsonian
Institution has perfected the details of common control of such stations
which it is expected may be established at an early day.