Plasma characteristics of the boundary layer in the Martian magnetosphere

Plasma and magnetic field data from circular orbits of the Phobos 2 spacecraft near Mars are examined to provide a description of the plasma properties of inner regions of the Mars magnetosheath and the boundary layer/plasma mantle. The data are analyzed in the VB coordinate system which is reasonable for draping magnetospheres of nonmagnetized planets and comets. It is shown that a boundary almost impermeable for protons is formed. The ion composition changes at this boundary, and a transition layer dominated by planetary ions is observed. The characteristics of the magnetosheath plasma is drastically changed near this ion composition boundary. A strong drop of the proton bulk velocity is accompanied by an increase of the proton temperature and intense fluxes of planetary ions. In dependence of the solar wind dynamic pressure and other factors, radius of the ''magnetospheric cavity'', virtually void of solar wind plasma varies from 4500 to 9500 km. During time intervals of very high solar wind dynamic pressure the cavity, divided up in lobe cells, is almost degenerated. The comparison of positions of different magnetospheric boundaries identified earlier from single-instrument measurements shows their collocation.