The New Student's Reference Work/Aurora Borealis
Aurora Borealis, often called Northern Lights, a luminous phenomenon of remarkable beauty occurring in the high latitudes. In intermediate latitudes the aurora most frequently presents the appearance of long streamers of pale yellowish light extending from the northern part of the horizon well nigh to the zenith. But in the higher latitudes this light appears frequently as an arch or even several arches, with the summits in the magnetic meridian. These streamers and arches are in almost constant motion, appearing to oscillate to and fro or to shoot suddenly upward and then to disappear with equal abruptness.
Since the auroras rotate with the earth, it is practically certain that they are phenomena which occur in the earth's atmosphere. And since they are almost universally accompanied by disturbances of the magnetic needle and by electrical disturbances, it seems highly probable that auroras are produced by electrical discharges, as was first suggested by Franklin. These discharges occur perhaps at a height of from 50 to 100 miles, where the atmospheric pressure does not amount to more than about one one-hundredth of an inch of mercury. Air under these conditions is a fairly good conductor of electricity.
When an aurora is examined with a prism, it presents an emission spectrum which is quite unique, consisting, as it does, of some half-dozen weak lines and one strong green line. This strong line has a wave length of 5,571 tenth-meters and apparently does not coincide with an equally strong line in any known substance. Such a spectrum indicates that auroras are in the condition of a glowing gas. And it is the opinion of two very high authorities, Vogel and Hasselberg, that the spectrum of the aurora is merely a modified spectrum of air, which as yet we have not been able to produce in the laboratory.
Contrary to the general impression, the frequency of auroral displays does not increase from equator to pole, but reaches a maximum at an average latitude of about 60°. So that the northern lights are not seen so frequently in Greenland and in Iceland as in regions south of these countries.
The name aurora borealis is due to Gassendi, who observed a brilliant display in France in 1621.
