Auroræ: Their Characters and Spectra. J. Rand Capron
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.
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.
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.
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.
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:—
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