The atmosphere is constantly charged with positive electricity,—electricity furnished by the vapors that rise from the sea, especially in tropical regions,—and, on the other hand, the earth is negatively electrized. The recomposition or neutralization of the two opposite electricities of the atmosphere and of the terrestrial globe is brought about by means of the moisture with which the lower strata of the air are more or less charged. But it is especially in the polar regions, where the eternal ice that reigns there constantly condenses the aqueous vapors under the form of haze, that this recomposition must be brought about; the more so, as the positive vapors are carried thither and accumulated by the tropical current, which, setting out from the equatorial regions, where it occupies the most elevated regions of the atmosphere, descends as it advances towards the higher latitudes, until it comes in contact with the earth in the neighborhood of the poles. It is there, then, chiefly, that the equilibrium between the positive electricity of the vapors and the negative electricity of the earth must be accomplished by means of a discharge, which, when of sufficient intensity, will be accompanied with light, if, as is almost always the case near the poles, and sometimes in the higher parts of the atmosphere, it take place among those extremely small icy particles which constitute the hazes and the very elevated clouds.
There can be no doubt that the occurrence of the phenomenon is materially dependent on the presence in the atmosphere of these particles of ice, forming a kind of thin haze, which, becoming luminous by the transmission of electricity, must appear simply as an illuminated surface of greater or less extent, and more or less cut up. The phenomenon actually takes place in this manner in the parts of the atmosphere that are the most distant from the earth. We perceive what are termed auroral plates of a purple or reddish-violet color, more or less extended, according as this species of veil, formed by icy particles, extends to a greater or less distance from the poles. The tenuity of this veil is such that it admits of our seeing the stars through the auroral plates. Of its existence, independently of indirect proofs, we have a direct demonstration in the observation of MM. Bixio and Baral, who, being raised in a balloon to a great height, found themselves, on a sudden, although the sky was entirely serene and the atmosphere cloudless, in the midst of a perfectly transparent veil, formed by a multitude of little icy needles, so fine that they were scarcely visible.
If we place the pole of an electro-magnet over the jets of electric light that are made to converge in extremely rarefied air, we shall see that the electric light, instead of coming out indifferently from all points of the upper surface, as had taken place before the magnetization, comes out from the points of the circumference only of this surface, so as to form around it a continuous luminous ring. This ring possesses a movement of rotation around the magnetized cylinder, sometimes in one direction, sometimes in another, according to the direction of the discharge and of the magnetization. Finally, some more brilliant jets seem to come out from this luminous circumference without being confounded with the rest of the group. Now the magnetic pole exercises over the luminous haze which we have mentioned as always present during an aurora precisely the same action which the pole of the electro-magnet exercises in the experiment just described; and what takes place on the small scale of the experiment is precisely what takes place on the large scale of the phenomenon of the aurora borealis.
The arc of the aurora borealis is a portion of a luminous ring, the different points of which are sensibly at equal distances from the earth, and which centres upon the boreal magnetic pole, so as to cut at right angles all the magnetic meridians that converge towards this pole. Such a ring, seen by an observer placed at the surface of the earth, evidently presents to him the known arc of the aurora; and its apparent summit is always necessarily situated in the magnetic meridian of the place.
The diameter of the luminous ring is greater in proportion as the magnetic pole is more distant from the surface of the earth, since this pole must be situated upon the intersection of the plane of the ring with the axis of the terrestrial globe; if we could determine rigorously the position of the aurora borealis, we should then have the means of knowing exactly that of the pole itself.
Each observer sees the summit of the auroral arc at his magnetic meridian; it is, therefore, only those who are on the same magnetic meridian who see the same summit, and who are able by simultaneous observations to take its height.
If the summit of the arc pass beyond the zenith of the observer, the latter is surrounded by the matter of the aurora borealis. This matter is nothing else than aqueous vapors traversed by the discharges, and which are in general luminous only at a certain height from the ground, either because the air is there more rarefied, or because they are themselves congealed, and more capable, consequently, of liberating their electric light. Then it is, that, from being nearer to the spot where the phenomenon is taking place, the observer hears the crepitation, or whizzing, of which we have spoken, especially if he be in an open country and in a quiet place. But if the arc do not attain to his zenith, he is situated beyond the region in which the meeting of the electric currents takes place; he sees only an arc a little more elevated to the north or the south, according as he is situated in one hemisphere or the other; and he hears no noise, on account of his too great distance. The crepitation is the result of the action of a powerful magnetic pole upon luminous electric jets in its immediate neighborhood. With regard to the sulphurous odor which some observers have perceived, it arises, as does that which accompanies the fall of lightning, from the conversion into ozone of the oxygen of the air, by the passage of electric discharges.
Gisler says, that on the high mountains of Sweden the traveller is sometimes suddenly enveloped in a very transparent fog, of a whitish-gray color inclining a little to green, which rises from the ground, and is transformed into an aurora borealis. The cirro-cumulus and the hazes become luminous when they are traversed by sufficiently energetic discharges of electricity, and when the light of day is no longer present to overcome their more feeble light. Dr. Usher describes an aurora borealis seen in the open day, at noon, May 24, 1778.
MM. Cornulier and Verdier are convinced, after carefully studying the subject, that there are almost always aurorae boreales in the high polar latitudes, and that their brilliancy alone is variable. This conviction is in accordance with the very careful observations which have now been made for four years in the northern hemisphere. It appears, as the result of these, that the aurora borealis is visible almost every clear night, but it does not show itself at all the stations at the same time. From October to March there is scarcely a night in which it may not be seen; but it is in February that it is most brilliant. In 1850 it was observed two hundred and sixty-one nights, and during 1851 two hundred and seven. The proportion of nights in which the aurora is seen is much greater the nearer we are to the magnetic pole.
De la Rive, from whose admirable treatise upon Electricity we have borrowed our general views, and whose theory we have attempted to illustrate in this paper, concludes that the aurora borealis is a phenomenon which has its seat in the atmosphere, and consists in the production of a luminous ring of greater or less diameter, having for its centre the magnetic pole. Experiment shows, as we have seen, that, on bringing about in rarefied air the reunion of the two electricities, near the pole of a powerful artificial magnet, a small luminous ring is produced, similar to that which constitutes the aurora borealis, and animated by a similar movement of rotation. The aurora borealis would be due, consequently, to electric discharges taking place in the polar regions between the positive electricity of the atmosphere and the negative electricity of the earth. These electric discharges taking place constantly, but with intensities varying according to the state of the atmosphere, the aurora borealis should be a daily phenomenon, more or less intense, consequently visible at greater or less distances, but only when the nights are clear,—which is perfectly in accordance with observation.