Seasonal variation of amplitude of geomagnetic sudden commencements near midnight at geostationary orbit

Abstract

Using digital magnetic data from the geostationary satellites of SMS/GOES series, amplitude of geomagnetic sudden commencements (SCs) observed near midnight was examined and a seasonal variation for it was confirmed. SC amplitude normalized by the value at Honolulu is smaller in Northern Hemisphere winter than in summer. Variations from a steady state before an SC to a quasi-steady state soon after the SC tend to be positive for horizontal and vertical components and inclination in summer, while those tend to be negative in winter. These variations seem to be related with the seasonal variation of position of the neutral sheet in the magnetospheric tail and a shift of the sheet at disturbed times. The direction of this shift is southward in summer and northward in winter such as the neutral sheet is nearer the solar magnetospheric equatorial plane (ecliptic plane). We calculate magnetic field caused by near-Earth tail currents on the assumption of a cylindrical magnetotail. The results show that both the magnetopause surface currents and the tail currents contribute to SC amplitude near midnight, and the seasonal variation is mainly caused by the variation of position and the shift of the neutral sheet.