A statistical study of the magnetosphere boundary crossings by the Geotail satellite

Geotail magnetometer observations from 1995-97 were used to identify 1369 dayside magnetospheric boundary crossings. Statistical properties of this set of multiple crossings are presented and discussed in terms of models for magnetopause motion. Our results indicate that equatorial boundary crossings are caused by north-south aligned ridges propagating azimuthally around the magnetosphere. Perpendicular orientations of the magnetospheric and magnetosheath magnetic fields inhibit the boundary motion. For the garden hose orientations of the interplanetary magnetic field (IMF), magnetospheric boundary motion is favoured in the dawn sector, while orthospiral orientations favour motion in the dusk sector. These results suggest that pressure pulses generated at the quasi-parallel bow shock drive the boundary motion.