AURORA BOREALIS (French, aurore borfale; German, Nordlicht), the northern light. An illumination in the sky, seen often est north of middle latitudes in the northern hemisphere, and south of them in the southern hemisphere. In our hemisphere it is generally visible in the north, and has, therefore, been called the Aurora Borealis. In the southern hemisphere it is called the Aurora Australis.
The frequency with which it is seen varies with the latitude of the place. It is compara tively rare within 45° of the equator, but be comes more frequent toward the north up to the latitudes of about 60°, where it sometimes becomes almost a nightly occurrence. Nearer the pole it again becomes less frequent. We shall first describe it as it is commonly seen in our own latitudes. The first noticeable phenomenon commonly occurs after the end of twilight, when the northern sky near the horizon will be seen illuminated with a light somewhat like that of the dawn. Careful examination will show, however, that the il lumination is in the form of a broad arch, highest near the magnetic north, and reaching the horizon in the northeast and northwest directions. Presently beams of light are seen crossing this arch with a quivering or flicker ing motion and shooting toward the zenith. Each beam constantly varies in brightness and seemingly fades away to give place to another.
In more northern latitudes, say north of 45° or 50°, these beams form the most brilliant feature of the aurora. Sometimes they are arranged in curved, wavy lines like the slats bf a window shade flying in the wind, giving the appearance of a scroll in the process of being unrolled. In the case of a very brilliant aurora the beams may cover almost or quite the entire sky. In this case they will be seen to converge toward a point commonly not far from the zenith. The appearance presented by the beams grows out of the direction in which they are seen and fron. the laws of perspective. Long-repeated observations show that the rays are really parallel to the direction of the dipping needle, or to the lines of the earth's magnetic force. In the latitudes which we have mentioned, the dip is commonly more than increasing to 90° at the magnetic pole; hence when a great number of beams, all parallel to each other, are viewed from a point on the earth's surface under the region in which they are found, they all seem to con verge according to the laws of perspective, toward that part of the sky to which the upper (south) pole of a dipping needle is directed. If the parallelism to the magnetic lines is ex act, the direction of this point should be the same as that of the compass needle itself. It is still an open question where the parallelism is exact. Many observations seem to show a deviation of 10° or more, hut the determina tion of the exact centre of convergence is diffi cult unless the rays are so numerous as to cover a large part of the sky, and it is not certain that the deviation may not be due to errors of estimation.
The Nature of the Auroral As a general, perhaps universal, rule, the rays or beams which we have described have a slightly yellowish tinge. When their light is analyzed with the spectroscope, many lines, in one case nearly 150, have been found in the spectrum. Of these the brightest and most constant is in the yellowish green part of the spectrum, hav ing a wave length of 557. This line is char acteristic of the aurora, and is almost certainly due to krypton. Several of the others have been identified with the lines of oxygen and of nitrogen, a result which might have been ex pected since it is practically certain that the aurora is due to an electrical discharge in or through our atmosphere.
The light of an aurora does not proceed wholly from the beams. Very irregular sheets of light, having the appearance of thin lumi nous clouds, are often seen. These are of various colors, red being especially frequent. The appearance is then that of a red cloud illuminated by the rays of the sun sometime after the latter has set. The light can, how ever, be easily distinguished from that of a cloud by its diffused .character and the ab sence of any definite outline. The height of the region in whioh the auroras are formed has never been definitely determined. The most important question is, whether the height is, in any case, above the upper limit of the atmosphere. This question is all the more difficult in that this limit is in itself an uncertain quantity. Observations of shooting stars show that these objects become visible at a height of about 100 miles above the earth's surface. The limit of the atmosphere must therefore be as high as this, and may be much higher. The difficulty of making observations upon the same auroral beams, at one moment, at different points of the region from which they are visible, is such that no exact deter mination of the height of a beam has ever been made. There is some reason to believe that the height may sometimes range from 100 to 150 miles, but there is no reason to believe that a beam is ever seen above the possible limit of the atmosphere. It might be supposed that by laying off a suitable base line on the earth, from the extremities of which the alti tudes of selected portions of the light are measured, the heights might be determined in a very simple manner, and indeed in the older meteorologies results of observations made in this way arc frequently stated. But it is now regarded as certain that such figures can claim but little accuracy. If the base is a long one, as is necessary in measuring the greater heights, it is very uncertain whether the two observers are measuring to the same region of illumina tion, beside which the very rapid motions of such regions in auroral display introduce a further element of uncertainty.