Magnetopause crossings observed at 6.6 RE

Thirty-five boundary crossing events, many with multiple crossings, have been observed in data from the GOES 6 and 7 geostationary satellites in the years 1989, 1990 and 1991. Many of the crossings within these events appear to have been caused primarily by inward boundary motion due to compression of the magnetosphere by the solar wind and some were crossings caused primarily by inward motion due to geomagnetic field erosion. Most crossings appear to be due to some combination of compression and erosion. A rise of the main, northward component of the Earth's magnetic field to approximately twice the dipole level, observed prior to a crossing, is taken as evidence of inward motion due to compression. When a crossing occurs with no prior rise of the northward component, it is taken as evidence of erosion. The magnetosheath field, just outside the boundary, is found to be predominantly southward for almost all of the crossings, including those due to compression as well as those due to erosion. This indicates that, even for the many events noted above for which inward motion by compression appears adequate to account for the boundary crossings, there is almost always some erosion present. In the 3-year interval, each spacecraft spent 0.3 % of the dayside observation time in the magnetosheath.