A study of geosynchronous magnetopause crossings

Sixty-four magnetopause crossings at geosynchronous orbit were detected by the GOES 2, 5, and 6 satellites between 1978 and 1986. The total accumulated crossing duration was 4 hours/year, or about 0.1% of the time. These rare events permit investigation of the interaction of the solar wind with the magnetosphere. This interaction has been examined through comparisons of solar wind and geophysical parameters during crossing episodes, and through superposed epoch event analysis of these periods. The study suggests that both high solar wind dynamic pressures and southward interplanetary Bz are required for the excursion of the magnetopause to geostationary orbit and beyond. Thus dynamic pressure alone is not sufficient to produce a magnetopause crossing; some erosion seems also to be required. An approximate relationship between the strength of the southward Bz and the subsolar distance to the magnetospheric boundary has been obtained: ΔR(RE) =0.08 Bz (nT), where ΔR is the change in standoff distance attributed to erosion. Further, magnetopause crossings are associated with the more intense, sudden-commencement geomagnetic storms. On average, storm sudden commencements occur 5 hours before a crossing, while an average B z turns southward 7 hours before the crossing. copyright American Geophysical Union 1989