Evidence of plasma vortices in the Venus plasma wake

Measurements conducted with plasma instruments in the Pioneer Venus Orbiter (PVO) and in the Venus Express (VEX) spacecraft have led to the observation of plasma velocity distributions that resemble the presence of vortices in the Venus wake. The velocity vectors of plasma fluxes measured along several orbits of the PVO trajectory reveal cases in which they are sunward oriented, and also instances where the velocity vectors are anti-sunward oriented but they point into the inner plasma wake. At the same time measurements conducted with the ASPERA-4 instrument of the VEX spacecraft show velocity vectors in the plasma wake that are arranged in patterns that are consistent with vorticity vector structures in the downstream vicinity of the polar regions and also in the Venus central wake. A fluid dynamic interpretation of such measurements in terms of vortex structures lead to calculations of the viscosity coefficient with estimates of the damping that they experience through local viscosity forces and are in agreement with those derived from the transport of solar wind momentum to the Venus upper ionosphere. From the analysis of the vorticity equation applied to the plasma conditions in the wake it is found that their evolution requires values of the viscosity coefficient that are comparable to those inferred from calculations of the momentum equation of the solar wind as it streams by the flanks of the Venus ionosheath, and thus the results obtained stress the value of viscosity as an important property of the plasma in the interaction of the solar wind with the Venus ionosphere.