The structure of the magnetopause is investigated by studying the interaction between two plasmas with solar wind and magnetospheric properties. Both Hall-MHD and hybrid (particle ions and fluid electrons) simulations are performed to compare and contrast the nature of the solutions in the fluid and the kinetic limits. It is shown that, in accordance with previous studies, the fluid solutions consist of multiple discontinuities and waves such as slow shocks and waves, as well as contact and rotational discontinuities. In contrast, the kinetic solutions consist of fewer discontinuities and include non-MHD boundaries. The difference between the two types of solutions are attributed to the absence of contact surfaces in collisionless plasmas and the possible Landau damping of slow waves. The kinetic solutions are found to be in a much better agreement with magnetopause observations, which have shown little evidence for the presence of slow shocks or contact discontinuities.

The results of kinetic calculations suggest that the presence of a small but finite normal component of the magnetic field allows for the mixing of magnetosheath and magnetospheric plasmas and may, in part, be responsible for the formation of the boundary layer even during the periods of northward interplanetary magnetic field. These results also show that the necessary changes in the fields and the plasma parameters across the magnetopause do not occur simultaneously (i.e., in the same spatial location). As such, the magnetopause cannot be viewed as a single boundary but instead is a region which includes both abrupt and gradual changes in the fields and plasma parameters. For example, the jump from the magnetosheath to the magnetospheric magnetic field strength takes place at a narrow region (a few ion inertial lengths), which we refer to as the magnetic boundary. This boundary, which has no MHD counter part, is not associated with any abrupt change in the plasma density or temperature.

Pile up of the magnetosheath mirror waves at the magnetic boundary can result in the compression and amplification of these waves. As a result, anticorrelated density and magnetic field fluctuations are present upstream of the magnetic boundary. Similar fluctuations have been observed at the magnetopause of the Earth and possibly at Jupiter and Saturn. Analysis of these waves at the Earth's magnetopause has shown that they have a finite frequency in the plasma rest frame. Here we show that this finite frequency is a direct consequence of the entry of the magnetosheath plasma into the boundary layer and may be used to determine the velocity of plasma penetration into the high magnetic field region. ¿ American Geophysical Union 1995