Bakerian lecture.―The aurora and its spectrum

Abstract

The beauty of the Polar Aurora, when it is seen in its full glory, has always deeply impressed those who have seen it, and has excited many to search for the true nature and cause of its occurrence. It occurs in both hemispheres, that in the northern being designated as Aurora Borealis , and that in the south as Aurora Australis . The frequency of occurrence varies with latitude and is greatest in the two zonal regions that make angles of about 20° with the earth's magnetic axis. Auroral displays are generally accompanied by magnetic storms, and a view commonly held is that both phenomena are due to streams of electrons emitted by the sun. Such streams, Prof. Birkland has shown, must necessarily describe paths on coming under the influence of the earth's magnetic field that follow closely the course of the streamers in an auroral display. The polar aurora appears with varying intensities and in various forms and colours. At times it is seen as a weak, diffuse, formless luminosity; at other times it appears as luminous draperies, bands or arcs, and often only as luminous beams or rays.