MHD Rankine—Hugoniot equations applied to earth's bow shock

Abstract

This paper compares computed results with upstream and downstream thermal pressures, and the downstream ion density and vector flow velocity measured by the Ames plasma probe on the Pioneer 6 spacecraft as earth's bow shock was traversed. The upstream ion density and vector flow velocity measured on Pioneer 6 by this experiment are used as independent variables, together with Pioneer 6 magnetic field measurements. MRD Rankine—Hugoniot equations for an isotropic plasma are used for these computations. Reasonable agreement is obtained between the measured and computed downstream ion density, thermal pressure and convective velocity magnitude and orientation, only when a 1γ change is made to a downstream magnetic field value used in the calculation. There is disagreement between several determinations of shock orientation and that deduced from coplanarity of the reported magnetic fields, and the shock orientation provides the co-ordinate system for solving the Rankine—Hugoniot equations. If a shock orientation determined by velocity coplanarity is used in the calculations, all computed quantities agree well with the experimental results. Other results suggest that a disagreement between computed and measured upstream thermal pressures may not be negligible in comparison with experimental uncertainties in upstream velocity and density. If the computations included anisotropic pressure terms and/or other factors such as plasma turbulence, better agreement might be obtained between computed and measured upstream thermal pressures.