2-DIMENSIONAL HYBRID SIMULATIONS OF IMPULSIVE PLASMA PENETRATION THROUGH A TANGENTIAL DISCONTINUITY

Two-dimensional hybrid simulations with particle ions and fluid electr ons are used to calculate the impulsive penetration process when a mag netosheath plasma irregularity interacts with the magnetopause. The ir regularity is modeled as a field-aligned filament with an excess densi ty impinging on the magnetopause. Simulations with strictly parallel o r antiparallel magnetic fields in the magnetosheath and in the magneto sphere are performed in order to investigate ion kinetic effects durin g the penetration of the plasma irregularity. The results show that a MHD approach is not appropriate when describing the impulsive penetrat ion of small-scale irregularities. If the fields in the magnetosheath and magnetosphere are antiparallel, the magnetic field gradient near t he magnetopause boundary leads to an asymmetric propagation of the mag netosheath plasma filament. In contrast, for moderate field changes ac ross the magnetopause the parallel case exhibits an almost straight-li ne propagation of the filament, while its main part is focused into a small region. In both cases, twin vortices are set up in the magnetosp here due to the filament motion. In the antiparallel case the shape of the filament is totally distorted by these vortices; these fluidlike structures stretch the magnetopause boundary and thereby increase the line to surface ratio of the boundary layer. The filament breaks in th e antiparallel as well as in the parallel case into smaller parts whic h leads to isolated islands of magnetosheath like plasma and field in the magnetosphere of the order of the ion gyroradius.