Waves on the dusk flank boundary layer during very northward interplanetary magnetic field conditions: Observations and simulation

The Geotail spacecraft made an inbound passage perpendicular to the dusk equatorial magnetopause on 1 August 1998 when the interplanetary magnetic field had been very northward for more than 10 hours. As the spacecraft moved through the low- latitude boundary layer, it detected waves with similar to 3 min period that caused transitions between cool dense magnetosheath plasma and mixed magnetosheath and magnetosphere plasmas. A region of highly fluctuating magnetic fields with variations up to 50 nT/ s was observed on leaving the magnetosheath plasma but not on its return to this region. This observation suggests that the boundary region is not laminar but more likely consists of Kelvin-Helmholtz vortices. Such fluctuations are probably caused by twisting of the field by the vortices, and they could be important in transporting plasma to the magnetosphere where densities of 5/ cc were observed. A global MHD simulation of the event reproduced the observed low- frequency waves, their counterclockwise polarization, and a region of high- velocity magnetosheath flow outside the magnetopause. This flow was associated with a decreased density and increased field strength which are characteristics of a plasma depletion layer. When Geotail moved further inward into the magnetosphere, it detected waves that are probably the magnetotail vortices which had been detected earlier by the ISEE spacecraft.