Goddard Space Flight Center, Greenbelt, Maryland 20771

For several hours on March 24, 1995, the Geotail spacecraft remained near the duskside magnetotail boundary some 15 R_E behind the Earth while the solar wind remained very quiet (V=330 km/s, n=14 to 21/cc) with a very steady 11 nT northward magnetic field. Geotail experienced multiple crossing of a boundary between a dense (n=19/cc), cool (T_P=40 eV), rapidly flowing (V=310 km/s) magnetosheath plasma and an interior region characterized by slower tailward velocities (V=100 km/s), lower but substantial densities (n=3/cc) and somewhat hotter ions (220 eV). The crossings recurred with a roughly 3 minute periodicity and all quantities were highly variable in the boundary region. The magnetic field, in fact, exhibited some of the largest fluctuations seen anywhere in space, despite the fact that the exterior magnetosheath field and the interior magnetosphere field were both very northward and nearly parallel. Based on an MHD simulation of this event, we argue that the multiple crossing are due to a Kelvin-Helmholz instability at the boundary that generates vortices which move past the spacecraft. It is concluded that the Kelvin-Helmholz instability is an important process for transferring energy, momentum and particles to the magnetotail during times of very northward IMF.

J. Geophys. Res., Vol. 105, No. A9, 21,159-21,173, 2000