The Trouvelot astronomical drawings manual

by E. L. Trouvelot

The auroral phenomena, although sometimes visible within the tropics, are, however, quite rare in these regions. For the most part they are confined within certain zones situated in high latitudes north and south. The zone where they are most frequent in our hemisphere forms an ellipse, which has the north pole at one of its foci; while the other is situated somewhere in North America, in the vicinity of the magnetic pole. The central line of the zone upon which the auroras seem to be most frequent passes from the northern coast of Alaska through Hudson's Bay and Labrador to Iceland, and then follows the northern coast of Europe and Asia. The number of auroras diminishes as the observer recedes from this zone, and it is only in exceptional cases that they are seen near the equator. Near the pole the phenomenon is less frequent than it is in the region described. In North America we occupy a favorable position for the observation of auroras, as we are nearer the magnetic poles than are the Europeans and Asiatics, and we consequently have a greater number of auroras in corresponding latitudes.

The position of the dark auroral segment varies with the place occupied by the observer, and its centre always corresponds with the magnetic meridian. In our Eastern States the auroral segment appears a little to the west of the north point; but as the observer proceeds westward it gradually approaches this point, and is due north when seen from the vicinity of Lake Winnipeg. At Point Barrow, in the extreme north-west of the United States, the aurora is observed in the east. In Melville Islands, Parry saw it in the south; while in Greenland it is directly in the west.

It is stated that auroras are more numerous about the equinoxes than they are at any other seasons; and also, when the earth is in perigee, than when it is in apogee. An examination which I have made of a catalogue by Professor Loomis, comprising 4,137 auroras observed in the temperate zone of our hemisphere from 1776 to 1873, sustains this statement. During this period, one hundred more auroras were recorded during each of the months comprising the equinoxes, than during any other months of the year; while eighty more auroras were observed when the earth was in perigee, than when it was in apogee. But to establish the truth of this assertion on a solid basis, more observations in both hemispheres will be required.

The aurora is not simply a terrestrial phenomenon, but is associated in some mysterious way with the conditions of the Sun's surface. It is a well-known fact that terrestrial magnetism is influenced directly by the Sun, which creates the diurnal oscillations of the magnetic needle. Between sunrise and two o'clock, the north pole of the needle moves towards the west in our northern hemisphere, and in the afternoon and evening it moves the other way. These daily oscillations of the needle are not uniform in extent; they have a period of regular increase and decrease. At a given place the daily oscillations of the magnetic needle increase and decrease with regularity during a period which is equal to 10⅓ years. As this period closely coincides with the Sun-spot period, the connection between the variation of the needle and these solar disturbances has been recognized.

Auroral phenomena generally accompany the extraordinary perturbations in the oscillations of the magnetic needle, which are commonly called "magnetic storms," and the greater the auroral displays, the greater are the magnetic perturbations. Not only is the needle subject to unusual displacements during an aurora, but its movements seem to be simultaneous with the pulsations and waving motions of the delicate auroral streamers in the sky. When the aurora sends forth a coruscation, or a streamer in the sky, the magnetic needle responds to it by a vibration. The inference that the auroral phenomena are connected with terrestrial magnetism is further supported by the fact that the centre of the corona is always situated exactly in the direction of that point in the heavens to which the dipping needle is directed.

It has been found that the aurora is a periodical phenomenon, and that its period corresponds very closely with those of the magnetic needle and Sun-spots. The years which have the most Sun-spots and magnetic disturbances have also the most auroras. There is an almost perfect similarity between the courses of the three sets of phenomena, from which it is concluded that the aurora is connected in some mysterious way with the action of the Sun, as well as with the magnetic condition of the earth.

A very curious observation, which has been supposed to have some connection with this subject, was made on Sept. 1st, 1859, by Mr. Carrington and Mr. Hodgson, in England. While these observers, who were situated many miles from one another, were both engaged at the same time in observing the same Sun-spot, they suddenly saw two luminous spots of dazzling brilliancy bursting into sight from the edge of the Sun-spot. These objects moved eastward for about five minutes, after which they disappeared, having then traveled nearly 34,000 miles. Simultaneously with these appearances, a magnetic disturbance was registered at Kew by the self-registering magnetic instruments. The very night that followed these observations, great magnetic perturbations, accompanied by brilliant auroral displays, were observed in Europe. A connection between the terrestrial magnetism and the auroral phenomena is further proved by the fact that, before the appearance of an aurora, the magnetic intensity of our globe considerably increases, but diminishes as soon as the first flashes show themselves.

The auroral phenomena are also connected in some way with electricity, and generate serious disturbances in the electric currents traversing our telegraphic lines, which are thus often rendered useless for the transmission of messages during great auroral displays. It sometimes happens, however, during such displays, that the telegraphic lines can be operated for a long distance, without the assistance of a battery; the aurora, or at least its cause, furnishing the necessary electric current for the working of the line. During auroras, the telephonic lines are also greatly affected, and all kinds of noises and crepitations are heard in the instruments.

Two observations of mine, which may have a bearing on the subject, present some interest, as they seem to indicate the action of the aurora on some of the clouds of our atmosphere. On January 6th, 1872, after I had been observing a brilliant aurora for over one hour, an isolated black cumulus cloud appeared at a little distance from the western extremity of the dark auroral segment. This cloud, probably driven by the wind, rapidly advanced eastward, and was soon followed by a succession of similar clouds, all starting from the same point. All these black clouds apparently followed the same path, which was not a straight line, but parallel to and concentric with the border of the dark auroral segment. When the first cloud arrived in the vicinity of the magnetic meridian passing through the middle of the auroral arc, it very rapidly dissolved, and on reaching this meridian became invisible. The same phenomenon was observed with the succession of black clouds following, each rapidly dissolving as it approached the magnetic meridian. This phenomenon of black clouds vanishing like phantoms in crossing the magnetic meridian, was observed for nearly an hour. On June 17th, 1879, I observed a similar phenomenon during a fine auroral display. About midway between the horizon and the polar star, but a little to the west of the magnetic meridian, there was a large black cumulo-stratus cloud which very slowly advanced eastward. As it progressed in that direction, its eastern extremity was dissolved in traversing the magnetic meridian; while, at the same time, several short and quite bright auroral rays issued from its western extremity, which in its turn dissolved rapidly, as if burned or melted away in the production of the auroral flame.

It seems to be a well observed fact, that during auroras, a strong sulphurous odor prevails in high northern latitudes. According to Dr. Hjaltalin, during these phenomena, "the ozone of the atmosphere increases considerably, and men and animals exposed out of doors emit a sulphurous odor when entering a heated room." The Esquimaux and other inhabitants of the northern regions assert that great auroras are sometimes accompanied by crepitations and crackling noises of various sorts. Although these assertions have been denied by several travelers who have visited the regions of these phenomena, they are confirmed by many competent observers. Dr. Hjaltalin, who has heard these noises about six times in a hundred observations, says that they are especially audible when the weather is clear and calm; but that when the atmosphere is agitated they are not heard. He compares them to the peculiar sound produced by a silk cloth when torn asunder, or to the crepitations of the electric machine when its motion is accelerated. "When the auroral light is much agitated and the streamers show great movements, it is then that these noises are heard at different places in the atmosphere."