THE DIURNAL BEHAVIOR OF SUDDEN COMMENCEMENTS OF MAGNETIC STORMS AT AGINCOURT

Abstract

The diurnal variations in the frequencies of sudden commencements of magnetic storms are examined, using the magnetograms from Agincourt for the period from 1946 to 1953 inclusive. The occurrence frequencies of sudden commencements exhibit an apparent diurnal tendency which has its minimum in the morning hours and maximum in the afternoon. The frequency distributions for several groups classified by their amplitudes, however, show different diurnal variations from one another. In order to confirm such local-time effects, a statistical analysis of the shape of sudden commencements has been made, and each sudden commencement classified according to its shape. Results show the following diurnal characteristics. A pronounced augmentation of the sudden commencement impulse occurs in the afternoon hours. In the morning the initial rise of sudden commencements is usually slight and they are frequently inverted in the H-trace, although some of them are quite clear in the D-trace. The local-time variation of the horizontal magnetic vectors at several stages of the initial phase is also estimated statistically. These results suggest that the diurnal control of the magnetic variation must be due to some additional field produced in the earth's upper atmosphere at the time of sudden commencement, tending to modify the primary cause.