Enhanced magnetospheric/boundary layer plasma flows observed during transient magnetopause crossings

We observed enhanced plasma flows inside the magnetopause while the Geotail satellite briefly crossed the magnetopause. The enhanced flows were mainly in the MN plane of the LMN coordinates. Some of them showed a bipolar signature, i.e., inward flow before the outbound (from the magnetosphere to the magnetosheath) crossing and then outward flow after the inbound (from the magnetosheath to the magnetosphere) crossing, in the component normal to the nominal magnetopause. We found two different types of the bipolar flow: one is roughly symmetric with respect to the center of the event, that is, the peak amplitudes of the inward and outward flows are comparable, and the other is strongly asymmetric, that is, the outward flow speed is much larger than the inward flow speed. Using a simple qualitative model, we show that the symmetric bipolar flow is consistent with a vortical plasma motion from the E × B drift. The source of the electric field E may be attributed to briefly compressed magnetopause moving tailward, which is induced by a transient external (solar wind/foreshock) pressure pulse. In the asymmetric case, the strong outward flows were accompanied by a depressed magnetic field strength. This suggests that the strong outward plasma motion is associated with transient magnetospheric expansion driven by external pressure pulse variations.