Magnetopause crossing of the geostationary satellite ATS 5 at 6.6<i>R_E</i>

Abstract

During the moderate magnetic storm of September 29–30, 1969, an unusually large magnetic field decrease preceded by an impulsive increase of about 100 γ was observed by the geostationary satellite ATS 5 at about 1733 UT on September 29. The field remained low for about 1 min and returned to the pre-event level as abruptly as it decreased. From the ATS 1 and ATS 5 observations and magnetograms from ground observatories, we inferred that the magnetosphere was greatly compressed before the event; the magnetopause distance was probably near 7 RE at the subsolar point. By comparing the changes observed by ATS 5 with the field measured by ATS 1, which was 3 hours behind ATS 5 in local time, we interpreted the event as a magnetopause crossing of ATS 5 caused by a localized rapid inward motion of the magnetopause and its subsequent recession, temporarily creating an indentation on the magnetopause surface and briefly exposing ATS 5 to the magnetosheath field. We suggest that the apparent ‘holes’ in the magnetic field observed by Ogo 3 and Ogo 5 in the magnetosphere near the magnetopause may have been caused, at least in some instances, by similar localized magnetopause motions.