Plasma measurements of the Pioneer Venus Orbiter in the Venus ionosheath - Evidence for plasma heating near the ionopause

A study of the angular spread of plasma fluxes detected in the Venus ionosheath is presented. It is shown that in the inner regions of the ionosheath, where the flux intensity is severely decreased with respect to solar wind values, the width of the azimuthal distribution of the local plasma is comparable to, or even larger than, that of the stronger fluxes measured in the outer ionospheath. The observed variation of the angular width suggests the existence of a source of heating near the ionopause but is not consistent with the overall cooling that would be expected if mass loading and charge exchange collisions were solely responsible for the interaction process at that boundary. Dissipative phenomene associated with local plasma turbulence processes seem to be required to account for the broad angular distributions seen near the ionopause.