AURORA POLARIS, a phenomenon of the atmosphere, sometimes of great beauty, known in the northern hemisphere as the aurora borealis, in the southern hemisphere as the aurora australis. Confining our attention to the aurora borealis, about which we have far more information, we find that the number of auroras increases as we go further north to certain limits, and then decreases as the pole is approached. For Europe and Asia the latitude of maximum frequency is +70°, for America +60°. This means that while an aurora is very rarely seen in southern Europe, auroras may often be seen in the central United States.
There are many types of auroral phenomena, sometimes sev eral types appearing simultaneously. These are known as arcs, rays, bands, curtains, draperies, coronas and diffuse glows. When the aurora is faint it is generally white; if fairly bright, yellowish; and when bright, many other colours, particularly red and green, appear. To the average American or European observer the most common forms are arcs and rays. The arc has its highest point on the magnetic meridian and frequently seems to reach to the horizon in the north-west and north-east. The under edge is more sharply defined, and as a consequence the sky below ap pears considerably darker than that above. When the arc is bright, the lower part will usually be red, the middle yellow and the upper part green. The arc may be visible alone, but fre quently from it rays appear to spread out like the spokes of a fan. The auroral rays sometimes are stationary, simply appear ing and disappearing without seeming to move. At other times there is a rapid cross motion and again they seem to shoot rapidly upward and then recede. These upward pulses are prob ably quite real, and may be due to progressive electric dis charges.
Further north the direction of the dip needle approaches nearer to the observer's zenith and the corona effect is seen, with rays spreading in all directions from this central ring of light, and even reaching to the horizon. This is perhaps the most striking and beautiful of all types of auroras. The curtains and draperies seem to be luminous clouds of light, usually of irregular outlines, that hang vertically in the sky. These are rarely seen in lower latitudes. The colours in the same aurora frequently change rapidly, even in the same area of the sky, but on the average are said to be more brilliant in the arctic regions.
The height of an aurora may be determined by the usual method of making simultaneous observations on a certain lu minous point therein from two stations whose distance apart is known. Such observations have been made with great success by photographing the aurora against a background of stars. By this method an upper limit of from 5o to 240m., and a lower limit of from 5o to room. have been fixed. These heights refer to auroras which occur in that portion of our atmosphere not illuminated by the sun. During the early part of the 20th cen tury rays have been measured in some cases to heights of from 24o to 48o miles. On Sept. 8, 1926, an unusual aurora was observed west of Norway. It appeared an arc to the eye, but the photographs showed it was composed of a curtain of rays. These rays extended as high as 3oom., but the diffuse form which ended the aurora reached the unexpected height of 600 miles. Calcu lation proved that this, as well as the rays in previous years which reached more than 24om. from the ground, were actually situated in that part of our upper atmosphere still illuminated by the sun. It seems proved therefore that sunlight has an ionizing action on the higher strata of our atmosphere, so that the illumination caused by the rays forming the aurora become visible at greater altitudes than when the light is withdrawn. These new heights for our atmosphere, which are derived from this work, open up most interesting questions as to how far it really does extend and what its physical properties must be at such great altitudes. The heights of meteors have long since proved that it was dense enough to cause them to become visible at least 120m. from the ground. Some phenomena of telescopic meteors may be explained by a height ten times as great, but other explanations of these phenomena are more probable. Thus auroral photographs give the first direct proof of such a great height for the atmosphere.
Trustworthy observers have, however, reported seeing the au rora between them and a mountain or cliff, or below clouds. This would mean an altitude of a mile or less. Also it is said to be accompanied by a crackling sound, like the rustling of silk. Such reports come from the far north where presumably the aurora would be nearer the ground. While difficult to explain, they have been made by so many observers that it is impossible arbitrarily to dismiss them as unreal.
In brightness different auroras vary immensely. Some are so faint as to be just visible, others exceed the light of the full moon, with all gradations between. A fairly good aurora might be com pared to the Milky Way in brilliancy. On the whole they become more brilliant toward the Arctic Circle. The auroral spectrum has been carefully studied and large numbers of characteristic lines, particularly a bright green line, are observed. Indeed by photographing the night sky, when to the eye no aurora is visible, it is possible to obtain this line. The inference is that there is a permanent aurora. In the ordinary auroral spectrum many nitro gen lines appear, and recently the famous green line has been proved definitely to be due to oxygen. The line in question has a wave-length of 5,577.34 Angstrom units. The early hypothesis that this line was due to krypton had long been held improbable. Frozen nitrogen particles had also been suggested.
From observations extending over more than two centuries the numbers of auroras have been found to follow rather closely the numbers of sun-spots (q.v.). These latter have a maximum approximately every I 1 years. It is further found that more au roras appear in March and September, when the earth is more directly opposite the spot zones on the sun, than in June and December when such is not the case. On the earth it is found that whenever a brilliant aurora appears there is almost sure to be a magnetic storm, accompanied by heavy earth currents. These latter seriously disturb telegraph and cable lines, sometimes put ting them out of commission for several hours. The magnetic storm is not necessarily confined to those areas over which the aurora is visible, but may extend over the rest of the earth as well. For the 19 great magnetic storms between 1875 and there was an average delay of 25 hours between the passage of the spots over the central meridian of the sun and the storm on the earth. Therefore we may infer with fair safety that the action, whatever its nature, was not propagated with the velocity of light. From what has been said it is clear that the appearance of an aurora must be closely connected with something that happens on the sun, and it is evident that the original outburst takes place on the sun and what is seen on the earth is a mere reflex action. That it is visible at all is due to the earth itself being an immense magnet surrounded by an atmosphere.
Since all bright shifting auroras are accompanied by magnetic storms, it seems certain that this type, and presumably all others, is caused by electric discharges of some kind. The current evi dently must come from or be induced by the sun. Two theories have been advanced. One is that the cause lies in negative par ticles shot off by that body and caught in the magnetic field of the earth. The second is that alpha-particles, with a plus charge, come to us from radioactive substances in the sun. In some cases at least it has been possible to prove that only plus charges could explain the accompanying magnetic effects, but it has been pointed out that even were these particles to move with the velocity of light they could not penetrate the atmosphere to within a distance of 16m. from the earth's surface. (C. P. 0.)