SOME GENERAL DESCRIPTIONS OF AURORÆ.
[Sidenote: Sir John Franklin’s description.]
Sir John Franklin (‘Narrative of a Journey to the Shores of the Polar Sea
in the years 1819, 1820, 1821, 1822’) describes an Aurora in these terms:—
[Sidenote: Parts of the Aurora: beams, flashes, and arches.]
“For the sake of perspicuity I shall describe the several parts of the
Aurora, which I term beams, flashes, and arches.
“The beams are little conical pencils of light, ranged in parallel lines,
with their pointed extremities towards the earth, generally in the
direction of the dipping-needle.
[Sidenote: Formation of the Aurora.]
“The flashes seem to be scattered beams approaching nearer to the
earth, because they are similarly shaped and infinitely larger. I have
called them flashes, because their appearance is sudden and seldom
continues long. When the Aurora first becomes visible it is formed like
a rainbow, the light of which is faint, and the motion of the beams
undistinguishable. It is then in the horizon. As it approaches the zenith
it resolves itself into beams which, by a quick undulating motion,
project themselves into wreaths, afterwards fading away, and again and
again brightening without any visible expansion or contraction of matter.
Numerous flashes attend in different parts of the sky.”
[Sidenote: Arches of the Aurora.]
Sir John Franklin then points out that this mass would appear like an
arch to a person situated at the horizon by the rules of perspective,
assuming its parts to be equidistant from the earth; and mentions a
case when an Aurora, which filled the sky at Cumberland House from
the northern horizon to the zenith with wreaths and flashes, assumed
the shape of arches at some distance to the southward. He then
continues:—“But the Aurora does not always make its first appearance as
an arch. It sometimes rises from a confused mass of light in the east
or west, and crosses the sky towards the opposite point, exhibiting
wreaths of beams or coronæ boreales on its way. An arch also, which is
pale and uniform at the horizon, passes the zenith without displaying any
irregularity or additional brilliancy.” Sir John Franklin then mentions
seeing three arches together, very near the northern horizon, one of
which exhibited beams and even colours, but the other two were faint and
uniform. (See example of a doubled arc Aurora observed at Kyle Akin,
Skye, Plate VII.)
He also mentions an arch visible to the southward exactly similar to one
in the north. It appeared in fifteen minutes, and he suggests it probably
had passed the zenith before sunset. The motion of the whole body of
the Aurora from the northward to the southward was at angles not more
than 20° from the magnetic meridian. The centres of the arches were as
often in the magnetic as in the true meridian. A delicate electrometer,
suspended 50 feet from the ground, was never perceptibly affected by the
Aurora.
[Sidenote: Aurora does not often appear until some hours after sunset.]
Sir John Franklin further remarks that the Aurora did not often appear
immediately after sunset, and that the absence of that luminary for some
hours was in general required for the production of a state of atmosphere
favourable to the generation of the Aurora.
[Sidenote: Aurora seen in daylight.]
On one occasion, however (March 8th, 1821), he observed it distinctly
previous to the disappearance of daylight; and he subsequently states
that on four occasions the coruscations of the Aurora were seen very
distinctly before daylight had disappeared.
[In the article “Aurora Polaris,” Encyc. Brit. edit. ix., the
Transactions of the Royal Irish Academy, 1788, are referred to, where Dr.
Usher notices that the Aurora makes the stars flutter in the telescope;
and that, having remarked this effect strongly one day at 11 A.M., he
examined the sky, and saw an Auroral corona with rays to the horizon.
Instances are by no means rare of the principal Aurora-line having
been seen in waning sunlight, and in anticipation of an Aurora which
afterwards appeared.]
[Sidenote: The Rev. James Farquharson’s observations. Auroral arch.
Passage across the zenith.]
The Rev. James Farquharson, from the observation of a number of
Auroræ in Aberdeenshire in 1823 (‘Philosophical Transactions,’ 1829),
concluded:—that the Aurora follows an invariable order in its appearance
and progress; that the streamers appear first in the north, forming an
arch from east to west, having its vertex at the line of the magnetic
meridian (when this arch is of low elevation it is of considerable
breadth from north to south, having the streamers placed crosswise in
relation to its own line, and all directed towards a point a little
south of the zenith); that the arch moves forward towards the south,
contracting laterally as it approaches the zenith, and increasing its
intensity of light by the shortening of the streamers and the gradual
shifting of the angles which the streamers near the east and west
extremities of the arch make with its own line, till at length these
streamers become parallel to that line, and then the arch is seen in
a narrow belt 3° or 4° only in breadth, stretching across the zenith
at right angles to the magnetic meridian; that it still makes progress
southwards, and after it has reached several degrees south of the zenith
again enlarges its breadth by exhibiting an order of appearances the
reverse of that which attended its progress towards the zenith from the
north; that the only conditions that can explain and reconcile these
appearances are that the streamers of the Aurora are vertical, or nearly
so, and form a deep fringe which stretches a great way from east to
west at right angles to the magnetic meridian, but which is of no great
thickness from north to south, and that the fringe moves southward,
preserving its direction at right angles to the magnetic meridian.
[Sidenote: M. Lottin’s observations.]
Dr. Lardner, in his ‘Museum of Science and Art,’ vol. x. p. 189 _et
seq._, alludes to a description of “this meteor” (_sic_) supplied by M.
Lottin, an officer of the French Navy, and a Member of the Scientific
Commission to the North Seas. Between September 1838 and April 1839,
being the interval when the sun was constantly below the horizon, this
savant observed nearly 150 Auroræ. During this period sixty-four were
visible, besides many concealed by a clouded sky, but the presence of
which was indicated by the disturbances they produced upon the magnetic
needle.
The succession of appearances and changes presented by these “meteors” is
thus graphically described by M. Lottin:—
[Sidenote: Formation of the auroral bow.]
“Between four and eight o’clock P.M. a light fog, rising to the altitude
of six degrees, became coloured on its upper edge, being fringed with the
light of the meteor rising behind it. This border, becoming gradually
more regular, took the form of an arch, of a pale yellow colour, the
edges of which were diffuse, the extremities resting on the horizon.
This bow swelled slowly upwards, its vertex being constantly on the
magnetic meridian. Blackish streaks divided regularly the luminous arc,
and resolved it into a system of rays. These rays were alternately
extended and contracted, sometimes slowly, sometimes instantaneously,
sometimes they would dart out, increasing and diminishing suddenly in
splendour. The inferior parts, or the feet of the rays, presented always
the most vivid light, and formed an arc more or less regular. The length
of these rays was very various, but they all converged to that point
of the heavens indicated by the direction of the southern pole of the
dipping-needle. Sometimes they were prolonged to the point where their
directions intersected, and formed the summit of an enormous dome of
light.
[Sidenote: It ascends to the zenith. Reaches the zenith.]
“The bow then would continue to ascend toward the zenith. It would
suffer an undulatory motion in its light—that is to say, that from
one extremity to the other the brightness of the rays would increase
successively in intensity. This luminous current would appear several
times in quick succession, and it would pass much more frequently from
west to east than in the opposite direction. Sometimes, but rarely, a
retrograde motion would take place immediately afterward; and as soon
as this wave of light had run successively over all the rays of the
Aurora from west to east, it would return in the contrary direction
to the point of its departure, producing such an effect that it was
impossible to say whether the rays themselves were actually affected by
a motion of translation in a direction nearly horizontal, or if this
more vivid light was transferred from ray to ray, the system of rays
themselves suffering no change of position. The bow, thus presenting
the appearance of an alternate motion in a direction nearly horizontal,
had usually the appearance of the undulations or folds of a ribbon or
flag agitated by the wind. Sometimes one, and sometimes both of its
extremities would desert the horizon, and then its folds would become
more numerous and marked, the bow would change its character and assume
the form of a long sheet of rays returning into itself, and consisting
of several parts forming graceful curves. The brightness of the rays
would vary suddenly, sometimes surpassing in splendour stars of the
first magnitude; these rays would rapidly dart out, and curves would be
formed and developed like the folds of a serpent; then the rays would
affect various colours, the base would be red, the middle green, and the
remainder would preserve its clear yellow hue. Such was the arrangement
which the colours always preserved. They were of admirable transparency,
the base exhibiting blood-red, and the green of the middle being
that of the pale emerald; the brightness would diminish, the colours
disappear and all be extinguished, sometimes suddenly and sometimes by
slow degrees. After this disappearance fragments of the bow would be
reproduced, would continue their upward movement and approach the zenith;
the rays, by the effect of perspective, would be gradually shortened;
the thickness of the arc, which presented then the appearance of a large
zone of parallel rays, would be extended; then the vertex of the bow
would reach the magnetic zenith, or the point to which the south pole of
the dipping-needle is directed. At that moment the rays would be seen in
the direction of their feet. If they were coloured they would appear as
a large red band, through which the green tints of their superior parts
could be distinguished, and if the wave of light above mentioned passed
along them their feet would form a long sinuous undulating zone; while
throughout all these changes the rays would never suffer any oscillation
in the direction of their axis, and would constantly preserve their
mutual parallelisms.
[Sidenote: Multiple bows. Corona formed.]
“While these appearances are manifested new bows are formed, either
commencing in the same diffuse manner or with vivid and ready formed
rays; they succeed each other, passing through nearly the same phases,
and arrange themselves at certain distances from each other. As many as
nine have been counted having their ends supported on the earth, and in
their arrangement resembling the short curtains suspended one behind the
other over the scene of a theatre, and intended to represent the sky.
Sometimes the intervals between these bows diminish, and two or more of
them close upon each other, forming one large zone traversing the heavens
and disappearing towards the south, becoming rapidly feeble after passing
the zenith. But sometimes also, when this zone extends over the summit of
the firmament from east to west, the mass of rays appear suddenly to come
from the south, and to form, with those from the north, the real boreal
corona, all the rays of which converge to the zenith. This appearance of
a crown, therefore, is doubtless the mere effect of perspective; and an
observer placed at the same instant at a certain distance to the north or
to the south would perceive only an arc.
“The total zone, measuring less in the direction north and south than in
the direction east and west, since it often leans upon the corona, would
be expected to have an elliptical form; but that does not always happen:
it has been seen circular, the unequal rays not extending to a greater
distance than from eight to twelve degrees from the zenith, while at
other times they reach the horizon.
“Let it then be imagined that all these vivid rays of light issue forth
with splendour, subject to continual and sudden variations in their
length and brightness; that these beautiful red and green tints colour
them at intervals; that waves of light undulate over them; that currents
of light succeed each other; and in fine, that the vast firmament
presents one immense and magnificent dome of light, reposing on the
snow-covered base supplied by the ground, which itself serves as a
dazzling frame for a sea calm and black as a pitchy lake. And some idea,
though an imperfect one, may be obtained of the splendid spectacle which
presents itself to him who witnesses the Aurora from the Bay of Alten.
[Sidenote: Duration of corona.]
“The corona when it is formed only lasts for some minutes; it sometimes
forms suddenly, without any previous bow. There are rarely more than two
on the same night, and many of the Auroras are attended with no crown at
all.
[Sidenote: Disappearance of Aurora.]
“The corona becomes gradually faint, the whole phenomenon being to
the south of the zenith, forming bows gradually paler and generally
disappearing before they reach the southern horizon. All this most
commonly takes place in the first half of the night, after which the
Aurora appears to have lost its intensity; the pencils of rays, the
bands, and the fragments of bows appear and disappear at intervals. Then
the rays become more and more diffused, and ultimately merge into the
vague and feeble light which is spread over the heavens, grouped like
little clouds, and designated by the name of auroral plates (plaques
aurorales). Their milky light frequently undergoes striking changes in
the brightness, like motions of dilatation and contraction, which are
propagated reciprocally between the centre and the circumference, like
those which are observed in marine animals called Medusæ. The phenomena
become gradually more faint, and generally disappear altogether on the
appearance of twilight. Sometimes, however, the Aurora continues after
the commencement of daybreak, when the light is so strong that a printed
book may be read. It then disappears, sometimes suddenly; but it often
happens that, as the daylight augments, the Aurora becomes gradually
vague and undefined, takes a whitish colour, and is ultimately so mingled
with the cirro-stratus clouds that it is impossible to distinguish it
from them.”
Lieutenant Weyprecht has grandly described forms of Aurora in Payer’s
‘New Lands within the Arctic Circle’ (vol. i. p. 328 _et seq._) as
follows:—
[Sidenote: Lieut. Weyprecht’s description. Formation of arches.]
“There in the south, low on the horizon, stands a faint arch of light.
It looks as it were the upper limit of a dark segment of a circle; but
the stars, which shine through it in undiminished brilliancy, convince
us that the darkness of the segment is a delusion produced by contrast.
Gradually the arch of light grows in intensity and rises to the zenith.
It is perfectly regular; its two ends almost touch the horizon, and
advance to the east and west in proportion as the arch rises. No beams
are to be discovered in it, but the whole consists of an almost uniform
light of a delicious tender colour. It is transparent white with a
shade of light green, not unlike the pale green of a young plant which
germinates in the dark. The light of the moon appears yellow contrasted
with this tender colour, so pleasing to the eye and so indescribable in
words, a colour which nature appears to have given only to the Polar
Regions by way of compensation. The arch is broad, thrice the breadth,
perhaps, of the rainbow, and its distinctly marked edges are strongly
defined on the profound darkness of the Arctic heavens. The stars shine
through it with undiminished brilliancy. The arch mounts higher and
higher. An air of repose seems spread over the whole phenomenon; here and
there only a wave of light rolls slowly from one side to the other. It
begins to grow clear over the ice; some of its groups are discernible.
The arch is still distant from the zenith, a second detaches itself from
the dark segment, and this is gradually succeeded by others. All now
rise towards the zenith; the first passes beyond it, then sinks slowly
towards the northern horizon, and as it sinks loses its intensity. Arches
of light are now stretched over the whole heavens; seven are apparent at
the same time on the sky, though of inferior intensity. The lower they
sink towards the north the paler they grow, till at last they utterly
fade away. Often they all return over the zenith, and become extinct just
as they came.
[Sidenote: Band of light appears. Second band and rays.]
“It is seldom, however, that an Aurora runs a course so calm and so
regular. The typical dark segment, which we see in treatises on the
subject, in most cases does not exist. A thin bank of clouds lies on
the horizon. The upper edge is illuminated; out of it is developed a
band of light, which expands, increases in intensity of colour, and
rises to the zenith. The colour is the same as in the arch, but the
intensity of the colour is stronger. The colours of the band change in
a never-ceasing play, but place and form remain unaltered. The band is
broad, and its intense pale green stands out with wonderful beauty on
the dark background. Now the band is twisted into many convolutions, but
the innermost folds are still to be seen distinctly through the others.
Waves of light continually undulate rapidly through its whole extent,
sometimes from right to left, sometimes from left to right. Then, again,
it rolls itself up in graceful folds. It seems almost as if breezes high
in the air played and sported with the broad flaming streamers, the ends
of which are lost far off on the horizon. The light grows in intensity,
the waves of light follow each other more rapidly, prismatic colours
appear on the upper and lower edge of the band, the brilliant white of
the centre is enclosed between narrow stripes of red and green. Out of
one band have now grown two. The upper continually approaches the zenith,
rays begin to shoot forth from it towards a point near the zenith to
which the south pole of the magnetic needle, freely suspended, points.
[Sidenote: Corona formed.]
“The band has nearly reached it, and now begins a brilliant play of rays
lasting for a short time, the central point of which is the magnetic
pole—a sign of the intimate connexion of the whole phenomenon with the
magnetic forces of the earth. Round the magnetic pole short rays flash
and flare on all sides, prismatic colours are discernible on all their
edges, longer and shorter rays alternate with each other, waves of light
roll round it as a centre. What we see is the auroral corona, and it
is almost always seen when a band passes over the magnetic pole. This
peculiar phenomenon lasts but a short time. The band now lies on the
northern side of the firmament, gradually it sinks, and pales as it
sinks; it returns again to the south to change and play as before. So
it goes on for hours, the Aurora incessantly changes place, form, and
intensity. It often entirely disappears for a short time, only to appear
again suddenly, without the observers clearly perceiving how it came and
where it went; simply, it is there.
[Sidenote: Single-rayed band.]
“But the band is often seen in a perfectly different form. Frequently
it consists of single rays, which, standing close together, point in an
almost parallel direction towards the magnetic pole. These become more
intensely bright with each successive wave of light; hence each ray
appears to flash and dart continually, and their green and red edges
dance up and down as the waves of light run through them. Often, again,
the rays extend through the whole length of the band, and reach almost
up to the magnetic pole. These are sharply marked, but lighter in colour
than the band itself, and in this particular form they are at some
distance from each other. Their colour is yellow, and it seems as if
thousands of slender threads of gold were stretched across the firmament.
A glorious veil of transparent light is spread over the starry heavens;
the threads of light with which this veil is woven are distinctly marked
on the dark background; its lower border is a broad intensely white band,
edged with green and red, which twists and turns in constant motion. A
violet-coloured auroral vapour is often seen simultaneously on different
parts of the sky.
[Sidenote: Aurora in stormy weather. Fragments.]
“Or, again, there has been tempestuous weather, and it is now, let us
suppose, passing away. Below, on the ice, the wind has fallen; but the
clouds are still driving rapidly across the sky, so that in the upper
regions its force is not yet laid. Over the ice it becomes somewhat
clear; behind the clouds appears an Aurora amid the darkness of the
night. Stars twinkle here and there; through the opening of the clouds we
see the dark firmament, and the rays of the Aurora chasing one another
towards the zenith. The heavy clouds disperse, mist-like masses drive on
before the wind. Fragments of the northern lights are strewn on every
side: it seems as if the storm had torn the Aurora bands to tatters,
and was driving them hither and thither across the sky. These threads
change form and place with incredible rapidity. Here is one! lo, it is
gone! Scarcely has it vanished before it appears again in another place.
Through these fragments drive the waves of light: one moment they are
scarcely visible, in the next they shine with intense brilliancy. But
their light is no longer that glorious pale green; it is a dull yellow.
It is often difficult to distinguish what is Aurora and what is vapour;
the illuminated mists as they fly past are scarcely distinguishable from
the auroral vapour which comes and goes on every side.
[Sidenote: Bands. Rays reach the pole. No noise.]
“But, again, another form. Bands of every possible form and intensity
have been driving over the heavens. It is now eight o’clock at night,
the hour of the greatest intensity of the northern lights. For a moment
some bundles of rays only are to be seen in the sky. In the south a
faint, scarcely visible band lies close to the horizon. All at once it
rises rapidly, and spreads east and west. The waves of light begin to
dart and shoot, some rays mount towards the zenith. For a short time it
remains stationary, then suddenly springs to life. The waves of light
drive violently from east to west, the edges assume a deep red and green
colour, and dance up and down. The rays shoot up more rapidly, they
become shorter; all rise together and approach nearer and nearer to the
magnetic pole. It looks as if there were a race among the rays, and that
each aspired to reach the pole first. And now the point is reached, and
they shoot out on every side, to the north and the south, to the east and
the west. Do the rays shoot from above downwards, or from below upwards?
Who can distinguish? From the centre issues a sea of flames: is that sea
red, white, or green? Who can say? It is all three colours at the same
moment! The rays reach almost to the horizon: the whole sky is in flames.
Nature displays before us such an exhibition of fireworks as transcends
the powers of imagination to conceive. Involuntarily we listen; such
a spectacle must, we think, be accompanied with sound. But unbroken
stillness prevails; not the least sound strikes on the ear. Once more
it becomes clear over the ice, and the whole phenomenon has disappeared
with the same inconceivable rapidity with which it came, and gloomy night
has again stretched her dark veil over everything. This was the Aurora
of the coming storm—the Aurora in its fullest splendour. No pencil can
draw it, no colours can paint it, and no words can describe it in all its
magnificence.”
A reproduction of the woodcut in Payer’s ‘Austrian Arctic Voyages,’
illustrating some of the features of the above description, will be found
on Plate I.
In the ‘Edinburgh Encyclopædia,’ article “Aurora,” we find:—
[Sidenote: Descriptions of Auroræ in high Northern latitudes.]
“In high Northern latitudes the Auroræ Boreales are singularly
resplendent, and even terrific.
“They frequently occupy the whole heavens, and, according to the
testimony of some, eclipse the splendour of stars, planets, and moon, and
even of the sun itself.
[Sidenote: In Siberia.]
“In the south-eastern districts of Siberia, according to the description
of Gmelin, cited and translated by Dr. Blagden (Phil. Trans. vol. lxxiv.
p. 228), the Aurora is described to begin with single bright pillars,
rising in the north, and almost at the same time in the north-east,
which, gradually increasing, comprehend a large space of the heavens,
rush about from place to place with incredible velocity, and finally
almost cover the whole sky up to the zenith, and produce an appearance
as if a vast tent were expanded in the heavens, glittering with gold,
rubies, and sapphires. A more beautiful spectacle cannot be painted; but
whoever should see such a northern light for the first time could not
behold it without terror.”
[Sidenote: Maupertius’s description at Oswer-Zornea.]
Maupertius describes a remarkable Aurora he saw at Oswer-Zornea on the
18th December, 1876. An extensive region of the heavens towards the south
appeared tinged of so lively a red that the whole constellation of Orion
seemed as if dyed in blood. The light was for some time fixed, but soon
became movable, and, after having successively assumed all the tints of
violet and blue, it formed a dome of which the summit nearly approached
the zenith in the south-west.
[Sidenote: Red Auroræ rare in Lapland.]
Maupertius adds that he observed only two of the red northern lights in
Lapland, and that they are of very rare occurrence in that country.
The observations of Carl Bock, the Norwegian naturalist, kindly
communicated by him to me, and detailed in Chapter III., quite confirm
this observation of Maupertius as to the rare occurrence of red Auroræ in
Lapland, he having only seen one.
[Illustration: Plate I.]