STRUCTURE OF THE MAGNETOPAUSE INFERRED FROM ONE-DIMENSIONAL HYBRID SIMULATIONS

The structure of the magnetopause is investigated by studying the inte raction between two plasmas with solar wind and magnetospheric propert ies, Both Hall-MHD and hybrid (particle ions and fluid electrons) simu lations are performed to compare and contrast the nature of the soluti ons in the fluid and the kinetic limits, It is shown that, in accordan ce with previous studies, the fluid solutions consist of multiple disc ontinuities and waves such as slow shocks and waves, as well as contac t and rotational discontinuities. In contrast, the kinetic solutions c onsist of fewer discontinuities and include non-MHD boundaries, The di fference between the two types of solutions are attributed to the abse nce of contact surfaces in collisionless plasmas and the possible Land au damping of slow waves, The kinetic solutions are found to be in a m uch better agreement with magnetopause observations, which have shown little evidence for the presence of slow shocks or contact discontinui ties. 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 par t, 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 para meters across the magnetopause do not occur simultaneously (i.e,, in t he same spatial location), As such, the magnetopause cannot be viewed as a single boundary but instead is a region which includes both abrup t 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 p lasma density or temperature, Pile up of the magnetosheath mirror wave s at the magnetic boundary can result in the compression and amplifica tion of these waves, As a result, anticorrelated density and magnetic field fluctuations are present upstream of the magnetic boundary, Simi lar fluctuations have been observed at the magnetopause of the Earth a nd possibly at Jupiter and Saturn, Analysis of these waves at the Eart h's magnetopause has shown that they have a finite frequency in the pl asma rest frame, Here we show that this finite frequency is a direct c onsequence 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.