Plasma boundary in planetary ionosheaths

Abstract

A description of the plasma and magnetic field conditions present within the shocked solar wind that streams around weakly magnetized planets is presented. The discussion is centered on an intermediate plasma transition that has been identified between the bow shock and the outer boundary of the effective obstacle of Venus and Mars. The changes seen in the plasma properties and in the geometry of the magnetic field across this intermediate transition are similar in both planets and suggest that equivalent processes may be operative in their region of interaction with the solar wind. In each case the intermediate plasma transition marks the outer boundary of a velocity shear that forms around the obstacle and develops within each planet's ionosheath. The available experimental information indicates that within the velocity shear the shocked solar wind experiences notable changes; while the local plasma temperature is high its density and the magnetic field intensity become significantly lower than in the outer ionosheath. In addition the ion population detected at or in the vicinity of the intermediate transition is dominated by particles of planetary origin which experience a local acceleration by the streaming flow.