Empirical Values of the Transport Coefficients of the Solar Wind: Conditions in the Venus Ionosheath

Abstract

Empirical values of the viscosity and thermal conductivity coefficients of the solar wind in the Venus ionosheath are inferred from measurements conducted with the Mariner 5 and the Pioneer Venus Orbiter spacecraft. Both coefficients are related through the width of the velocity and temperature boundary layers seen at the flanks of the Venus ionosheath and imply Pr ≈ 0.7 values for the Prandtl number of the solar wind in that region. The viscosity coefficient has been derived by estimating the relative value of the viscous and magnetic forces in the momentum equation of the super-Alfvenic solar wind, and the thermal conductivity coefficient is obtained from the energy equation by comparing changes in the velocity and temperature across the boundary layers. An estimate of the latter coefficient follows from the excess heating that is implied by the enhanced temperatures measured with the Mariner 5 in the inner ionosheath. In this case, thermal dissipation is associated with the transport of solar wind momentum to the polar upper ionosphere, and thus it is related to the plasma channels that extend downstream from the magnetic polar regions.