Two-dimensional hybrid simulations with particle ions and fluid electrons are used to calculate the impulsive penetration process when a magnetosheath plasma irregularity interacts with the magnetopause. The irregularity is modeled as a field-aligned filament with an excess density impinging on the magnetopause. Simulations with strictly parallel or antiparallel magnetic fields in the magnetosheath and in the magnetosphere are performed in order to investigate ion kinetic effects during the penetration of the plasma irregularity. The results show that a MHD approach is not appropriate when describing the impulsive penetration of small-scale irregularities. If the fields in the magnetosheath and magnetosphere are antiparallel, the magnetic field gradient near the magnetopause boundary leads to an asymmetric propagation of the magnetosheath plasma filament. In contrast, for moderte field changes across the magnetopause the parallel case exhibits an almost straight-line propagation of the filament, while its main part is focused into a small region.

In both cases, twin vortices are set up in the magnetosphere 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 the antiparallel as well as in the parallel case into smaller parts which leads to isolated islands of magnetosheath like plasma and field in the magnetosphere of the order of the ion gyroradius. ¿ American Geophysical Union 1994