Flowers of the Sky

by Richard A. Proctor

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.

Fig. 9.—The Northern Magnetic Meridians and Lines of Equal Dip.

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.