Geotail observations of the Kelvin-Helmholtz instability at the equatorialmagnetotail boundary for parallel northward fields

For several hours on March 24, 1995, the Geotail spacecraft remained near t he duskside magnetotail boundary some 15 R-E behind the Earth while the sol ar wind remained very quiet (V=330 km s(-1), n=14-21 cm(-3)) with a very st eady 11-nT northward magnetic field. Geotail experienced multiple crossings of a boundary between a dense (n=19 cm(-3)), cool (T-p=40 eV), rapidly flo wing (V=310 km s(-1)) magnetosheath plasma and an interior region character ized by slower tailward velocities (V=100 km s(-1)), lower but substantial densities (n=3 cm(-3)) and somewhat hotter ions (220 eV). The crossings rec urred with a roughly 3-min periodicity, and all quantities were highly vari able 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 bot h very northward and nearly parallel. On the basis of an MHD simulation of this event, we argue that the multiple crossings are due to a Kelvin-Helmho ltz instability at the boundary that generates vortices which move past the spacecraft. A determination of boundary normals supports Kelvin-Helmholtz theory in that the nonlinear steepening of the waves is seen on the leading edge of the waves rather than on the trailing edge, as has sometimes been seen in the past. It is concluded that the Kelvin-Helmholtz instability is an important process for transferring energy, momentum and particles to the magnetotail during times of very northward interplanetary magnetic field.