Viscous flow in the near-Venusian plasma wake

Abstract

The acquisition of the Venera 9 and 10 plasma probe data has provided further support for the notion that the shocked solar wind in the flank regions of the Venusian ionosheath interacts viscously with the local ionospheric plasma and that the resulting mixing region is rapidly forced into the planetary umbra. The measured velocities and flow directions suggest that the deflection of the mixing layer downstream of the terminator proceeds at a fast pace, as was predicted from the analysis of a suitable MHD analog proposed by Perez-de-Tejada and Dryer (1976). By measuring the apparent displacement of the mixing region within the planetary umbra with respect to the position of the undeviated ionopause (calculated from the inviscid flow model of Spreiter et al. (1970)) it is possible to obtain an order-of-magnitude estimate of the equivalent kinematic viscosity coefficient of the flow in the region. Some speculative comments are also given in regard to the behavior of the flow in the Venusian plasma wake, in particular, to the implications that the penetration of the mixing layer into the umbra may have for the formation of the nighttime Venusian ionosphere.