After discussing auroral phenomena in general, including types, latitude variation, periodicity, height, and cause, this article describes the principal features of the remarkably brilliant aurora of March 7–8, 1918, and presents a large number or detailed accounts by observers in the United States.
A chart of the United States shows places from which reports of the aurora were received. To facilitate intercomparison the descriptions have been grouped and discussed by latitude belts of 2½° each, for as would be expected, the display was pretty much the same at the same latitudes.
The aurora became visible at dusk, March 7 and attained its greatest brilliance, generally, at 9:30 p. m. (90th Meridian Time). Since no display is homogeneous, however, there are variations in the times of greatest brilliance and in the appearance of details of the display, although there is general unanimity concerning the times, colors, brilliance, and aspect, among widely scattered observers. The descriptions of the positions of arches, particularly of a prominent red one, make it obvious that the actual location of the aurora is the factor which determines its aspect, and the distance to which it can be seen, while the lack of streamers in the display at most southern points show that the clearness of the air limits its visibility.
It seems that the magnetic disturbance accompanying this aurora was a repetition, after three 27-day intervals, of the large magnetic storm of December 16–17, 1917. There was a considerable disturbance on January 12, and a minor one at the end of the next 27-day interval in February. Auroras on April 4, 5, and 6, marked the end of this strong series, which was probably caused by 5 successive presentations of an active area on the sun.