Evidence for a viscous boundary layer at the Venus ionopause from the preliminary Pioneer Venus results

A comparative study of the preliminary Pioneer Venus results is presented and its implications discussed in connection with the viscous flow interpretation of the ionosheath plasma at the Venusian terminator. It is shown that the various preliminary observations strongly support the case for a flattened ionopause boundary near the terminator as would be expected from the viscously induced deflection of solar wind fluxes in that region. Considerations of mass flux conservation and transfer of momentum are used to prepare an empirical formulation from which estimates of the transport properties of the ionosheath flow can be made. It is suggested that the position of the ionopause downstream from the terminator is not necessarily characterized by a balance condition between the ionospheric thermal pressure below and the magnetic pressure above but that the thermal and kinetic energy densities of the local ionosheath fluxes may be directly responsible for its location.