The magnetopause at 5.2 RE in August 1972: Magnetopause motion

The magnetopause passed ATS 5 at 6.6 RE and Explorer 45 at 5.2 RE, both near local noon, several times late on August 4, 1972. These passages occurred during an interval of unusually high solar wind velocity and high interplanetary magnetic field. There were several times when the magnetopause moved inward by compression of the magnetosphere. There were also times when the compressed field decreased by flux erosion. During one 25-min interval both satellites, separated by 3 RE, were observing the same magnetosheath field structure, moving past in the solar wind flow. During three of the Explorer 45 magnetopause passages a reversal of the field, from north to south, was observed with a minimum north-south component in the center of the magnetopause. In each case the magnetosphere field was north and the magnetosheath field was south, both being several hundred gammas in amplitude. During the first approach of the magnetopause to Explorer 45, 20-γ pulsations in field magnitude with a 2-min period were observed. They appeared not to be due to surface waves propagating in the magnetopause.