The Aurora Borealis
By Claude E. Benson.
“Science tells us,” I wrote in a previous number,[1] “that the cyclones, typhoons, hurricanes, “tornadoes and such-like atmospheric commotions are but the faint reproductions of gales of “a scale and velocity unimaginably greater in the upper air.” The above sentence is merely a corollary to the suggestion that Science is continually telling us things which appear to be in direct contradiction to the evidence of our senses. For instance, in the Northern Hemisphere we live in a region of prevailing southerly, south-westerly and westerly airs, but the mass of the atmosphere, Science tells us, is moving steadily in the contrary direction, viz., southwards, in the teeth, so to speak, of the prevailing breezes. When it reaches the Equator it walks upstairs several miles and returns to the Pole in a hurry. A cyclone, in fact, is merely the base of an enormous vortex in an aerial river northward bound at express speed.
As a matter of fact, so far as I can make out, the great motor forces of the Earth all seem to be worked on the same principle: an endless processional cycle, from the Poles to the Equator, from the Equator to the Poles. This is the law of the atmosphere, of the ocean, and especially of that most subtle, most manageable ‑ and unmanageable ‑ of all forms of energy, electricity, which steals along the Earth’s surface to the Equator, and there, rising into the upper air, returns steadily, but with lurid intervals, to the Poles: to descend, sometimes unseen, sometimes in the flickering splendours of the Aurora.
To most Ramblers, surely, the Aurora is known not merely by hearsay. They must have seen it in the Highlands, in the Northern Playground, possibly on “Iceland’s greasy mountains,” probably, in the case of one adventurous Rambler, on the wastes of distant Siberia. Probably, also, the first sight of its wondrous loveliness I elicited from them words of admiration; but in my case it was the reverse ‑ very much so. I was night-fishing in Scotland ; the conditions were ideal, and I was confidently promising myself for breakfast a dish to make the mouth of Lucullus water ‑ fresh-run herling, when out came, that beastly Aurora and down went every fish with its neb against the stones at the bottom, and refused to stir a fin, lured I never so cunningly.
A few nights later I was caught by a thunderstorm which put down the fish quite as effectually, but at that time I should never have dreamed of connecting a flash of lightning with an auroral display, and yet it would seem certain that the two phenomena are intimately associated.
From my youth up, of course, I had known that Auroræ had been regarded as supernatural appearances. The Esquimaux believe them to be the spirits of their ancestors playing with the skull of a walrus. The northern peasant would know
By the streamers that shot so bright
That spirits were riding the Northern Light.
Like eclipses, they have been regarded as portentous of calamity. Thus Aytoun sings how before the battle of Flodden:
All night long the Northern Streamers
Shot across the trembling sky;
Fearful lights that never beckon,
Save when kings or heroes die.
That desperate conflict does indeed seem to have been I preceded by some unaccountable precurse of fierce events, but I cannot help thinking the last two lines of the quatrain were suggested by a greater than Aytoun –
The heavens themselves blaze forth the death of princes.
Be that as it may, if the theory is correct, northern royalties and others who mock at fear and are not affrighted must have rather an uncomfortable time, in spite of their blood and heroism, during periods of exceptional auroral activity.
I determined, however, to have grounds more relative than barbarous superstition or poetic fancy, and turned to Modern Science. Sir Robert Ball put me on the right track.
“We have still to note,” he writes, “an extraordinary feature which points to an intimate connection between the phenomena of sun spots and the purely terrestrial phenomena of magnetism. It has often been noticed that a maximum abundance of sun spots occurs simultaneously with an unusual amount of disturbance of the magnetic needle, and . . . that an abnormal development of the earth’s magnetism is accompanied on the very day of the storm by a marked disturbance of the sun’s surface. It also appears that the sun spots are specially abundant about three or four days before the outbreak, though on the day immediately preceding, the sun may seem perfectly quiescent. The Earth’s magnetism is well known to be connected with the phenomena of the Aurora Borealis, inasmuch as an unusual Aurora seems to be invariably accompanied by a great magnetic disturbance. It has also been shown that there is an almost perfect parallelism between the intensity of auroral phenomena and the abundance of sun spots.”
The Earth, it appears, is a gigantic revolving magnet. Along the surface from the Poles to the Equator creeps an unceasing current of electricity which is constantly being thrown off or expelled into the air as positive electricity, the discharge ever increasing in volume as it approaches the Equator, where it reaches its maximum. Once up in the air the fluid, to use a convenient term, finds itself rather in a tight corner. It cannot rise above a certain height because the thin upper atmosphere will not conduct it, and it cannot return to the earth owing to certain atmospheric conditions and the opposition of the upward discharge of electricity; consequently, it streams backwards towards the Poles, seeking the line of least resistance to return to Mother Earth; this it finds along the line of the dip of the Magnetic Needle. There is, therefore, near the Magnetic Pole, where the Needle stands on its head, a constantly descending stream of electricity, sometimes invisible, sometimes luminous. When the whole or portion of this electric fall becomes visible, an Aurora is formed; an Aurora is, in fact, diffused lightning, returning quietly to earth.
This process, however, does not go on quite evenly. The upper and lower currents constantly vary in volume and intensity; very often, especially in the Tropics, the Earth throws off more electricity than the upper air can contain. There is no escape for the congested fluid upward, and the route to the Poles is already filled to bursting ‑ and burst it does. The pent-up electricity forces; its way through all opposition back to earth again in flame and tumult. In other words, we have a thunderstorm.
There appears to be no doubt that the regular descending flow of electricity, the sphere of auroral activity, forms a circle round the earth near the Poles. Not very long ago a scientist, by a very ingenious experiment, created round the head of a revolving magnet a small circle of magnetic light ‑ made, in fact, a miniature Aurora in his own laboratory. The elevation of the great auroral ring, above the Earth’s surface seems largely a matter of conjecture. Scientists give about 150 miles as somewhere near the mark, but there can be no doubt that portions sometimes descend quite close to the earth.
The general form of the Aurora is a low arch of streaks of light enclosing a dark space, the former of such extreme tenuity that the stars can be seen shining through. The streaks are of amazing variety of colour and incalculable swiftness. It is quite impossible for the eye to follow the movement of the lances of light; it is only conscious of a gleam when it is gone and its place taken by another of different colour and brilliancy, and one is finally content to gaze in delighted bewilderment, These dazzling displays, it would seem, take place in the denser atmosphere near the earth’s surface; in the higher, thinner air, the Aurora is more sober in hue and less eccentric in behaviour.
The beautiful phenomenon of the Corona, a luminous ring, with long, distinct rays, very slender but extremely bright, diverging from it in all directions, is not, as is generally supposed, any peculiar combination of the auroral structure. To see a,Corona you must be in a certain position, your eye in a line with the Magnetic Zenith with which the Auroral Zenith invariably coincides. This is a hard saying, but I will try and explain. No one can look up into a cupola ‑ the dome of St. Paul’s, for example unless he is somewhere under the dome, i.e., in the right position to do so. In the same way, no one can see a Corona unless he is in the right position to do so ‑ under the dome, as it were, with the auroral rays converging on his line of vision. Now a Corona never lasts long ‑ a few seconds at most, and then it seems to burst like a firework, scattering sparks in all directions, but this is only an illusion. It has, in fact, moved, and the stationary eye is no longer in the right place. As a matter of fact, the auroral band is never still; it oscillates; it swings from north to south during the day, and northwards again at night ‑ and its arc is sometimes colossal, twelve degrees, or thereabouts.
Whether the Aurora hisses and crackles or is noiseless is still a vext question. Scientists who have explored the Polar regions and lived side by side with the Aurora for months and months all agree that the display is perfectly silent; trappers, Indians and Esquimaux are unanimous in maintaining that it does fizz. At one time they had the support of Science. A careful observer in Paris ‑ and the Aurora has been seen as far south as Baghdad, and once in the middle of last century the entire globe, except the Tropics, was said to be wrapped in Auroræ ‑ a careful observer in Paris noted an Aurora, and, at the same time, was sensible of a light crackling sound, like the flickering of flames, which he was confident proceeded from the phenomenon. This would be strong evidence, did we not remember that the speed of sound is 1,100 feet per second, and of light something like 187,000 miles per second, so that it appears there must be some flaw in the Parisian scientist’s calculation. Nevertheless, it does appear to me ‑ and this suggestion is wholly unauthoritative ‑ that, as thunder is caused by the clashing together of the containing atmospheric walls of the vacuum created by the lightning flash, so that the Aurora, quâ diffused lightning, might conceivably bring about an attenuated, and, under certain conditions, audible thunder.
The accepted explanation I understand to be that the murmuring sounds which accompany the Aurora proceed from the Polar ice. Anyone who stands near a considerable sheet of frozen water on a still night will be conscious of gentle, whispering, creaking sounds; and, if he examine the edge in the morning, will find that the ice has been cracked and split in many directions. The fissures are often exceedingly minute, it is true, but the cleaving of them is sufficient to account for the gentle creakings heard during the night. Now on the vast icefields of the Polar regions this process is going on unceasingly on a gigantic scale. We have indisputable evidence that these great ice sheets are never quite quiet. Sometimes the rending of the ice, when violent, is accompanied by stupendous ruin it and clamour; sometimes, when gentle and gradual, by low hissings and cracklings. But there is silence never.