Most accounts of the aurora describe the typical appearances that it assumes as seen from a single place on the surface of the earth, but say little, if anything, about the form of the phenomenon as a whole or about its position in space. We shall follow a different plan here, and ask the reader, first of all, to imagine himself viewing the aurora from a point some thousands of miles away from our planet.
The solar emission above mentioned, when sufficiently intense, produces in the upper atmosphere a glow like that seen in a vacuum tube when an electrical discharge passes through it. From our vantage point in outer space we shall notice that this glow is not spread over the whole globe, but forms two rings, which encircle the polar regions of both hemispheres, though neither the geographic nor the magnetic poles lie at their centers. The rings do not extend down into the lower atmosphere, but hang about 60 miles above the earth’s surface.
The reason for this segregation of the aurora in high latitudes is that the earth is a great magnet, and magnets have the power of deflecting an electrical discharge in their vicinity. An appearance much resembling the two auroral zones of the earth was produced, on a small scale, by the late Professor Kr. Birkeland of Christiania, who magnetized a metal globe and allowed an electrical discharge to play upon it in a vacuum. The surface of the globe was coated with a phosphorescent substance, which glowed under the discharge in two rings, corresponding roughly to those of the aurora. In both cases the discharge follows what are called the magnetic “lines of force.” Our earth, like other magnets, is enveloped and penetrated by such lines. At any point on the earth the direction of the neighboring lines of force is shown by the dipping needle, which assumes a position parallel to them. At a magnetic pole the needle points straight up and down, and everywhere in high latitudes it has a position not very much inclined to the vertical, while in low latitudes it is more or less horizontal.
If, now, for the sake of simplicity we confine our attention to the northern hemisphere, and imagine ourselves maintaining our watch for months and years together, we shall discover that much of the time there is no ring to be seen; at other times there may be a small or partial ring; and occasionally there is a very broad, conspicuous ring, spreading so far south that it overlies the northern part of the United States and most of Europe. Evidently the emission from the sun that causes the auroral discharge varies greatly in strength, and this is in accordance with what we know about solar activities in general.
Next let us take a closer look at the ring, whether from outer space or from the earth’s surface. We shall find that it is made up, at least in part, of a multitude of luminous beams directed out into space and undergoing rapid changes in position and form. These beams, which really mark out the streams of the discharge in the upper air, follow the lines of force. In high latitudes they are nearly vertical with respect to the underlying surface of the earth. Even in the United States (when the aurora extends so far south) they are much more nearly vertical than horizontal. A dipping needle will show, at any place, just how they should stand.
Photographic Analysis of Lightning. This double photograph was made by Dr. B. Walter of Hamburg, the leading expert in the field of lightning photography. The picture at the left was taken with a stationary camera. The photograph at the right, taken at the same time with a revolving camera, shows that one of the main flashes (the one to the right) was a single discharge, and the other a multiple discharge.
From any distant point on the earth’s surface either north or south of it, the visible portion of the auroral ring presents the appearance of an arch across the horizon. Arctic explorers, far within the Arctic Circle, see this arch to the south of them. In our latitudes it spans the northern horizon. Separate beams or streamers may be distinguishable or not, according to the brightness of the discharge or its distance from the point of observation. Combinations of beams constitute so-called “draperies.”
Photograph of Lightning, Showing “Black” Flashes. (graph by F. Ellerman.)