_THE AURORA BOREALIS._
Among the objects in view, when the recent Polar expedition was fitted
out, was the hope that during the winter of 1875-76 the scientific
observers who accompanied the expedition might be able to study the
Aurora Borealis under unusually favourable conditions. This hope was, as
most of my readers doubtless know, disappointed. Few auroras were seen,
and those seen were not remarkable either for brilliancy or for beauty
of colour. Yet in the very disappointment of the hope which had been
entertained on this subject there was very significant evidence
respecting the aurora, as will presently be shown. The quiescence, at
that time, of the forces which produce the auroral streamers had its
meaning, and a very strange one.
The aurora is one of those phenomena of nature which are characterized
by exceeding beauty, and sometimes by an imposing grandeur, but are
unaccompanied by any danger, and indeed, so far as can be determined, by
any influence whatever upon the conditions which affect our well-being.
Comparing the aurora with a phenomenon akin to it in
origin--lightning--we find in this respect the most marked contrast.
Both phenomena are caused by electrical discharges; both are exceedingly
beautiful. It is doubtful which is the more imposing so far as visible
effects are concerned. When the auroral crown is fully formed, and the
vault of heaven is covered with the auroral banners, waving hither and
thither silently, now fading from view, anon glowing with more intense
splendour, the mind is not less impressed with a sense of the wondrous
powers which surround us than when, as the forked lightnings leap from
the thundercloud, the whole heavens glow with violet light, and then
sink suddenly into darkness. The solemn stillness of the auroral display
is as impressive in its kind as the crashing peal of the thunderbolt.
But there is a striking contrast between the feelings with which we
regard the safe splendours of the aurora and the terrible glory of the
lightning flash. One display we contemplate with the calmness engendered
by absolute security; the other--no matter how little the fear of death
may affect the reason--cannot be regarded without exciting the
consciousness of danger. We witness in safety, so far as itself is
concerned, the flash whose light illuminates the cloud masses above and
around us, but for aught we know it may be the last we shall ever see,
since no man killed by lightning ever saw the flash which brought his
death.
I do not purpose to consider here at any length those facts respecting
the aurora which properly find their place in text-books of science, but
those only which are less commonly dealt with, and seem at once most
suggestive and most perplexing.
The reader is no doubt aware that auroras, or polar streamers, as they
are sometimes called, are appearances seen not around the true poles of
the earth, but around the magnetic poles, which lie very far away from
those geographical poles which our arctic and antarctic seamen have in
vain attempted to reach. We in England, though much nearer to the north
pole than the inhabitants of Canada, see far fewer auroras than they do,
and those we see are far less splendid, simply because we are farther
away from the northern magnetic pole. This will be seen from the
accompanying pair of maps (from my "Elementary Physical Geography"),
showing where the northern and southern magnetic poles lie. Again, you
will see from the northern map, that from England the northern magnetic
pole lies towards the west of due north. That is why when we see a fully
developed auroral arch in this country its crown lies towards the west
of north (almost midway between north and north-west). I may have
occasion at another time to consider the curious changes which affect
the actual position of the magnetic poles and lines; in this place I
merely note that what is now said respecting them only refers to the
present time.
[Illustration: Fig. 9.--The Northern Magnetic Meridians and Lines of
Equal Dip.]
[Illustration: Fig. 10.--The Southern Magnetic Meridians and Lines of
Equal Dip.]
The formation of auroral streamers around the magnetic poles of the
earth shows that these lights are due to electrical discharges, just as
the general magnetic phenomena of the earth indicate the existence of
electrical currents. The earth, in fact, with its envelope of air, moist
and dense near the surface, rare and dry above may be regarded as an
enormous magnetic instrument, a core surrounded by conducting matter, in
which electrical currents pass whenever the condition of the earth's
magnetism changes. The discharges of electricity, though only visible at
night, take place in reality in the daytime also. According to their
extent and position, varying with the varying conditions under which
they take place, their aspect changes. Moreover, from different parts of
the earth the appearance of the aurora is different. From low latitudes
(I speak now of magnetic latitudes as indicated by the closed curves
around the magnetic poles in the maps), the auroral arch is seen towards
the north in our hemisphere, towards the south in the other hemisphere.
From points nearer the magnetic pole it is seen overhead, and when that
pole is approached still nearer, the crown of the arch is seen on the
side remote from the pole,--that is, towards the south in our
hemisphere, towards the north in the southern hemisphere.
Remembering that the aurora is due to electrical discharges in the upper
regions of the air, it is interesting to learn what are the appearances
presented by the aurora at places where the auroral arch is high above
the horizon,--these being, in fact, places nearly _under_ the auroral
arch. M. Ch. Martins, who observed a great number of auroras at
Spitzbergen in 1839, thus writes (as translated by Mr. Glaisher)
respecting them: "At times they are simple diffused gleams or luminous
patches; at others, quivering rays of pure white which run across the
sky, starting from the horizon as if an invisible pencil were being
drawn over the celestial vault; at times it stops in its course, the
incomplete rays do not reach the zenith, but the aurora continues at
some other point; a bouquet of rays darts forth, spreads out into a fan,
then becomes pale, and dies out. At other times long golden draperies
float above the head of the spectator, and take a thousand folds and
undulations as if agitated by the wind. They appear to be but at a
slight elevation in the atmosphere, and it seems strange that the
rustling of the folds as they double back on each other is not audible.
Generally, a luminous bow is seen in the north; a black segment
separates it from the horizon, the dark colour forming a contrast with
the pure white or bright red of the bow, which darts forth rays,
extends, becomes divided, and soon presents the appearance of a luminous
fan, which fills the northern sky, and mounts nearly to the zenith,
where the rays, uniting, form a crown, which in its turn darts forth
luminous jets in all directions. The sky then looks like a cupola of
fire; the blue, the green, the yellow, the red, and the white vibrate in
the palpitating rays of the aurora. But this brilliant spectacle lasts
only a few minutes; the crown first ceases to emit luminous jets, and
then gradually dies out; a diffused light fills the sky; here and there
a few luminous patches, resembling light clouds, open and close with
incredible rapidity, like a heart that is beating fast. They soon get
pale in their turn, everything fades away and becomes confused, the
aurora seems to be in its death-throes; the stars, which its light had
obscured, shine with a renewed brightness; and the long polar night,
sombre and profound, again assumes its sway over the icy solitudes of
earth and ocean."
The association between auroral phenomena and those of terrestrial
magnetism has long been placed beyond a doubt. Wargentin in 1750 first
established the fact, which had been previously noted, however, by
Halley and Celsius. But the extension of the relation to phenomena
occurring outside the earth--very far away from the earth--belongs to
recent times.
The first point to be noticed, as showing that the aurora depends partly
on extra-terrestrial circumstances, is the fact that the frequency of
its appearance varies greatly from time to time. It is said that the
aurora was hardly ever seen in England during the seventeenth century,
though the northern magnetic pole was then much nearer to England than
it is at present. Halley states that before the great aurora of 1716
none had been seen (or at least recorded) in England for more than
eighty years, and no remarkable aurora since 1574. In the records of the
Paris Academy of Sciences no aurora is mentioned between 1666 and 1716.
At Berlin one was recorded in 1707 as a very unusual phenomenon; and the
one seen at Bologna in 1723 was described as the first which had ever
been seen there. Celsius, who described in 1733 no less than three
hundred and sixteen observations of the aurora in Sweden between 1706
and 1732, states that the oldest inhabitants of Upsala considered the
phenomenon as a great rarity before 1716. Anderson, of Hamburg, states
that in Iceland the frequent occurrence of auroras between 1716 and 1732
was regarded with great astonishment. In the sixteenth century, however,
they had been frequent.
Here, then, we seem to find the evidence of some cause external to the
earth, as producing auroras, or at least as tending to make their
occurrence more or less frequent. The earth has remained to all
appearance unchanged in general respects during the last three
centuries, yet in the sixteenth her magnetic poles have been frequently
surrounded by auroral streamers; during the seventeenth these streamers
have been seldom seen; during the last two-thirds of the seventeenth
century auroras have again been frequent; and during the present century
they have occurred sometimes frequently during several years in
succession, at others very seldom.
Let us inquire a little more closely into the circumstances attending
auroral displays, in order to ascertain what external cause it is which
thus influences their occurrence.
Connected as auroras are with the phenomena of terrestrial magnetism,
we may expect to find some help in our inquiry from the study of these
phenomena.
Now it appears certain that magnetic phenomena are partly influenced by
changes in the sun's condition. We may well believe that they are in the
main due to the sun's ordinary action, but the peculiarities which
affect them seem to depend on _changes_ in the sun's action. It is found
that the daily oscillation of the magnetic needle corresponds with the
diurnal change in the position of the sun owing to the earth's rotation.
An annual change affecting that oscillation depends on the varying
distance of the sun as the year proceeds. The daily change is not only
greater than the annual, but is characterized by irregularities, when
the face of the sun shows the greatest number of spots. It was found by
General Sabine, says Mr. Balfour Stewart, "that the aggregate value of
magnetic disturbances at Toronto attained a maximum in 1848, nor was he
slow to remark that this was also Schwabe's period of maximum sun-spots.
It was afterwards found, by observations made at Kew, that 1859 (another
of Schwabe's years) was also a year of maximum magnetic disturbance....
There is also some reason to believe that on one occasion our luminary
was caught in the very act. On the first of September, 1869, two
astronomers, Carrington and Hodgson, were independently observing the
sun's disc, which exhibited at that time a very large spot, when, about
a quarter past eleven, they noticed a very bright star of light suddenly
break out over the spot and move with great velocity across the sun's
surface. On Mr. Carrington sending afterwards to Kew Observatory, at
which place the position of the magnet is recorded continuously by
photography, it was found that a magnetic disturbance had broken out at
the very moment when this singular appearance had been observed." The
dip of the magnetic needle, its deflection from the north, the
inferiority of its directive force, were all three simultaneously and
abruptly altered, and continued so for many hours.
Nor are we left in any doubt as to the connection between such
well-marked disturbances of the magnetic needle. While the needle was
thus violently displaced, vivid auroras occurred over the greater part
of both the northern and southern (magnetic) hemispheres. They were seen
in latitudes where usually auroras are as infrequent as rain in
Peru,--at Rome, in the West Indies, even within eighteen degrees of the
equator.
The disturbance of the earth's electrical condition was well shown in
other ways. Mr. C. V. Walker, the telegraphist, found that strong
electrical currents affected the various telegraphic lines throughout
England. These currents changed in direction every two or three minutes.
In many places it was impossible to send telegraphic messages. In
America some of the signalmen received severe electric shocks. "At a
station in Norway," says Sir J. Herschel, "the telegraphic apparatus was
set fire to; and at Boston, in North America, a flame of fire followed
the pen of Bain's electric telegraph (which writes down the message upon
chemically prepared paper)."
Many of my readers will doubtless remember the auroras of May 13, 1869,
and October 24, 1870, both of which occurred when the sun's surface was
marked by many spots, and both of which were accompanied by remarkable
disturbance of the earth's magnetism.
It may, then, fairly be assumed that the occurrence of auroras depends
in some way, directly or indirectly, on the condition of the sun. But
what the real nature of that connection may be is not to be easily
determined. It is clear that the eleven-year-period of sun-spots is not
the only, or even the chief period affecting auroras, for we have seen
that sometimes for a full century, or even more, very few auroras are
seen. It is not by any means certain that the connection between the
sun's condition and the occurrence of auroras is of the nature of cause
and effect; quite probably sun-spots and auroras depend on some common
cause as yet undetected,--and possibly never to be detected by man.
Regarding the auroral streamers as terrestrial lights only, but in some
sense like the light reflected by planets in having their real source
in the sun, we can no longer speak, as Humboldt was wont to do, of our
planet possessing a power of emitting light of its own. Yet his manner
of dealing with auroral light still possesses interest for us,
especially in relation to the question whether these polar lights are
emitted by other planets and may possibly be discerned from our earth.
"It results from the phenomena of the aurora," said Humboldt, "that the
earth is endowed with the property of emitting a light distinct from
that of the sun. The intensity of this light is rather greater than that
of the moon in its first quarter. It is at times, as on January 7, 1831,
strong enough to admit of one's reading printed characters without
difficulty. This light of the earth, the emission of which towards the
poles is almost continuous" (this, however, is not strictly the case),
"reminds us of the light of Venus, the part of which not lighted by the
sun often glimmers with a dim phosphorescent light. Other planets may
also possess a light evolved out of their own substance."
I would venture, however, to express strong doubts as to the possibility
of discerning, either on Venus or on any other planet, the auroral
gleams which may very probably illuminate at times their nocturnal
skies. It must be remembered that the aurora, when at its brightest and
covering a large part of the sky, only gives about as much light as the
moon in her first quarter,--that is, as one half of a disc so small that
180,000 such discs would not equal the entire sky. The luminosity of the
aurora is then in reality very small; probably far less than that of the
earth's surface when illuminated by the full moon. A distant hill on
which the rays of the full moon are falling seems strongly illuminated,
and yet its light is really so faint that we could scarcely discern it
at all save for the favouring effect of contrast. We know this, because
we often see portions of the moon's surface which are illuminated by
earthshine (when we see what is called the old moon in the new moon's
arms), and these portions are quite faint by comparison with the rest of
the moon; yet earthshine exceeds moonshine at least twelve times, and
probably more nearly twenty times in splendour.
The glimmering phosphorescent light, supposed to have been seen on parts
of Venus not lighted by the moon, is a phenomenon about which
experienced telescopists are somewhat doubtful, though Webb speaks of
the appearance as remarkably well attested, quoting, amongst others, the
following cases. In 1715, Derham, in his "Astro-Theology," says that
"the sphericity or rotundity is manifest in our moon, yea, and in Venus,
too, in whose greatest falcations" (_i.e._, when they appear as
crescents) "the dark parts of their globes may be perceived, exhibiting
themselves under the appearance of a dull and rusty colour." In 1806,
the phenomenon displayed itself beautifully to Harding three times and
to Schröter once within five weeks. "Guthrie and others noticed it a few
years ago, with small reflectors, in Scotland; Purchas, at Ross, in
England; De Vico and Palomba, many times in Italy." Winnecke records a
similar observation, though very faint, 1871, September 25, a little
before noon. Van Hahn also says he saw it repeatedly, by day as well as
by night, and with several instruments; he was, however, an inferior
observer. The dark side is sometimes described as grey, sometimes as
reddish. The phenomenon has, on the other hand, been looked for
specially, on several occasions, by practised observers, using very fine
instruments, who have failed to recognise any trace of it.
One of the most remarkable observations ever made on Venus must here be
mentioned. Mädler states that on one occasion, when he was observing the
planet, he saw a number of brushes of light diverging from the circular
side (_i.e._, the outside of the planet's crescent), lasting as long as
the planet could be seen that evening, and remaining unchanged when he
changed the position of the telescopic eye-piece, or used a different
one. "He attempts no explanation," says Webb, "but thinks it could not
have been an optical illusion. This is certainly _possible_, but it is
an instructive instance of the oversights which may be incidental even
to great philosophers, that it never seems to have occurred to him to
try another telescope!" It cannot be doubted that the evidence would
have been greatly strengthened had he changed telescope as well as
eye-piece; though it is not readily to be explained how a known
telescope, frequently used as well before as after this strange
appearance was seen, could for one evening only have played so strange a
trick as Mädler's must have done, if what he saw was merely an
instrumental illusion.
However, whether we have telescopic evidence or not respecting auroral
lights surrounding the polar regions of other planets, we can have very
little doubt that some among the planets, if not all of them, resemble
our earth in this as in so many other respects. The aurora is a cosmical
phenomenon, not one peculiar to our own earth. It is not, indeed,
altogether certain that our sun himself may not be girt round by mighty
auroral streamers, and that the light of these may not constitute a
noteworthy portion of the corona of glory seen around him during the
time of total eclipse.
This view, indeed, although it has not been definitely entertained as I
have here expressed it, has been suggested by reasoning which led others
to suppose that the coloured prominences around the sun may be auroras.
Perceiving the nature of the connection between terrestrial magnetism
and auroras, Balfour Stewart reasoned that we may extend our inquiries
and ask, "If the sun's action is able to create a terrestrial aurora,
why may he not also create an aurora in his own atmosphere?" It occurred
independently to General Sabine, Prof. Challis, and himself, that the
red flames visible during a total solar eclipse "may be solar auroræ."
We now know that the solar flames are not auroræ, nor, properly
speaking, flames at all, but great masses of glowing vapour. It is not,
however, by any means so clear that the solar corona is not auroral in
its nature. The following reasoning, applied by Balfour Stewart to the
sun's prominences, applies with much greater force to the corona. After
mentioning the height (from 70,000 to 80,000) which some prominences
attain, he proceeds, "Considering the gravity of the sun, we are
naturally unwilling to suppose that there can be any considerable amount
of atmosphere at such a distance from his surface; and we are therefore
induced to seek for an explanation of these red flames amongst those
phenomena which require the smallest possible amount of atmosphere for
their manifestation. Now the experiments of Mr. Gassiot and the observed
height of the terrestrial aurora alike convince us that this meteor
will answer our requirements best. And besides this, the curved
appearance of these red flames, and their high actinic power, in virtue
of which one of them, not visible to the eye, was photographed by Mr. De
la Rue, are bonds of union between these and terrestrial auroræ."
All this and much more may be said of the solar corona. Its streamers
extend not 70,000 or 80,000 miles, but 700,000 or 800,000 miles, from
the surface of the sun, where the pressure must be far smaller than near
the summits of even the loftiest prominences. They are curved and
striated, like those of the aurora, whereas the shapes of the
prominences bear only a distant resemblance to auroral streamers. They
possess a high actinic (_i.e._, photographic) power, as is shewn by the
readiness with which, during the total eclipse of December, 1871, they
were photographed, no less than six well-defined negatives being taken
both by Col. Tennant, at Ootacamund, and by Mr. Davis, at Baikal, during
the brief continuance (only a few minutes) of total obscuration. In
every respect the solar corona accords far better than do the solar
coloured prominences with the appearance we should expect to recognise
in solar auroras.
In particular, it has always seemed to me that the curved, especially
the doubly curved, streamers of the corona can only be well explained
by regarding the corona as in the main an auroral phenomenon. If mighty
currents prevailed in the higher regions of a rare atmosphere, extending
hundreds of thousands of miles from the sun's surface, appearances such
as these curved streamers would undoubtedly be explained. But no one who
considers the effect of the sun's tremendous attractive power on such an
atmosphere can fail to perceive that, according to the known laws
connecting gaseous pressure and density, the density of that atmosphere
would be enormously great, even at a very great distance from the sun's
surface, if the curved streamers really were caused by atmospheric
currents. We know, on the contrary, from the behaviour of comets which
have passed very near to the sun, that the atmosphere above his visible
surface must be very rare indeed.
It must not be understood, however, that I regard the corona as simply a
great solar aurora. It is certain that the whole region filled by the
corona is occupied by immense numbers of scattered meteors, and
extremely probable that large quantities of cometic matter exist within
the same region. Vaporous masses may also be there, circling
independently around the sun. But that this region is illuminated
constantly by auroral light, varying greatly in intensity and position,
seems very strongly indicated by all that we know about the corona, as
seen during different total eclipses of the sun.
If we so viewed the solar corona, and found our earth, therefore, in
this respect resembling the great central orb of the solar system, we
could not but regard as extremely probable the theory that other planets
also resemble the central body in this respect. We might then picture to
ourselves every orb in the solar system carrying onward its faintly
luminous crowns of boreal and austral light, not shining with constant
lustre, or in the same constant position, but at one time leaping in
coloured steamers to a great distance from the body they adorned, and
anon sinking down and growing fainter and fainter, or occasionally
disappearing altogether. Then, when some great disturbance affected the
central sun, and caused _his_ auroral banners to shine out more
brilliantly and to attain a greater extension, suddenly the auroral
streamers of all the planets would leap out into new light and life,
playing around the northern and southern magnetic poles of those orbs,
even as electric brushes play around the positive and negative
electrodes of a Geissler's tube. "Suddenly" at least so far as each
planet is concerned, but not suddenly throughout the whole system. For
the magnetic influences, like the light and heat of the sun, require
time for their transmission. Yet, so rapidly do they travel that, in a
few hours, the auroral illumination would extend from the central sun to
the outermost limits of his system.
It remains that I should make a few remarks on the evidence which that
wonderful instrument of research, the spectroscope, has afforded
respecting the light of the aurora.
Angström was the first to observe the spectrum of the aurora borealis.
He found that the greater part of the auroral light, as observed in
1867, was of one colour, yellow, but three faint bands of green and
greenish blue colour were also seen. The aurora of April 15, 1869, was
seen under very favourable conditions in America. Prof. Winlock,
observing it at New York, found its spectrum to consist of five bright
lines, of which the brightest was the yellow line just mentioned. One of
the others seems to agree very nearly, if not exactly, in position with
a green line, which is the most conspicuous feature of the spectrum of
the solar corona. During the aurora of October 6, 1869, Flögel noticed
the strong yellow line and a faint green band. Schmidt, on April 5,
1870, made a similar observation. He saw the strong yellow line, and
from it there extended towards the violet end of the spectrum a faint
greenish band, which, however, at times showed three defined lines,
fainter, than the yellow line.
It was not till the magnificent aurora of October 24, 25, 1870, that any
red lines were seen in the spectrum of an aurora. On that occasion the
background of auroral light was ruddy, and on the ruddy background there
were seen three deep red streamers very well defined. The ruddy
streamers, on the night of October 25, converged towards the auroral
crown, which was on that occasion singularly well seen. Förster of
Berlin failed to see any red line or band despite the marked ruddiness
of the auroral light. But Capron at Guildford saw a faint line in the
red part of the spectrum; and Elger at Bedford observed a red band in
the light of the red streamers, the band disappearing, however, when the
spectroscope was directed on the white rays of the aurora.
As yet the auroral spectrum has not been interpreted. It is not a
spectrum which can be (at present) artificially produced. We understand
the spectrum of the sun and stars, because spectra of the same order can
be produced in our laboratories. The spectra of the planets, so far as
they differ from the spectrum of reflected sunlight in showing signs of
the absorptive action of the planetary atmosphere, have been similarly
interpreted. So also the spectra of the coloured solar prominences are
understood, while those of nebulæ and comets, though not as yet
thoroughly explained, have been partly interpreted, because of their
partial agreement with the known spectra of earthly elements. But as yet
neither the spectrum of the aurora nor that of the solar corona has been
explained. The reason probably is, that the conditions under which the
light of the aurora as of the corona is formed are not such as have been
or perhaps can be attained or even approached in laboratory
experiments.
[Illustration]