GAS-DYNAMIC MODELING OF THE VENUS MAGNETOTAIL

A gasdynamic, convected magnetic field model of the solar wind interaction with Venus is used for the first time to model the steady state Venus magnetotail. Model results are directly compared with observations. The flow obstacle surface is approximated as a tangential discontinuity. The obstacle shape is an input parameter to this model. An initial obstacle shape, accurate on the dayside, is defined by balancing a hydrostatic equilibrium approximation for the internal plasma pressure with an external flow pressure approximation. These pressure approximations produce a cylindrical obstacle in the distant tail. A refined obstacle shape that attempts to balance this same internal pressure with the calculated external flow pressure tapers inward toward the tail axis downstream of the terminator. Cold fluid (photoionized planetary oxygen) is added to the flow about the tapered model obstacle. The resultant bulk plasma flow and magnetic field properties compare well with experimentally observed average proton velocity and magnetic field components in the magnetotail. The added oxygen plasma has significant number densities only within 1 R(V) of the tail axis in the distant tail. The model predicts central magnetotail oxygen plasma number densities of about 0.2 cm-3 and temperatures on the order of 10(6)-degrees-K, flowing tailward at speeds as low as 200 m/s. These properties are consistent with the flat, featureless Pioneer Venus Orbiter plasma analyzer spectra observed in the deep central tail. Pickup ions, in the test particle limit, match direct observations of tail pickup ions. These steady state model results suggest that the mass addition at Venus originating above the dayside ionopause is predominantly fluidlike and produces the slowed flows and severe field draping observed in the central distant tail. Oxygen ions produced higher above the ionopause on the dayside, at much lower number densities, behave more as test particles. Their large gyroradii produce an asymmetric population in the distant outer tail and sheath.