Magnetic field near Venus: A comparison between Pioneer Venus Orbiter magnetic field observations and an MHD simulation

Pioneer Venus Orbiter (PVO) measurements revealed the shape and the changing location of the Venus bow shock with solar cycle and provided a detailed picture of the magnetic field pileup in the dayside magnetosheath. Nevertheless, the reason for the increase of the terminator shock position to the observed distances has evaded our understanding, and the "magnetic barrier" region has been studied primarily by comparisons with gasdynamic models due to the difficulty of using more sophisticated treatments. In this study we investigate the extent to which a three-dimensional magnetohydrodynamic (MHD) model of the Venus-solar wind interaction, with and without "mass loading" by photoionization of the atomic oxygen upper atmosphere, can reproduce some of the basic features of the dayside magnetic field observed on PVO. The ideal MI-ID model uses a conducting sphere to represent the basic Venus ionospheric obstacle to the solar wind flow. We adopt the viewpoint that during solar maximum, a conducting obstacle with oxygen mass loading is appropriate, while a no-mass loading case is a good first approximation to the solar minimum situation. The MHD simulations are found to give a realistic picture of both the shape of the bow shock and its observed elliptical cross section at the terminator. The introduction of the oxygen mass loading moves the shock position to that observed at solar maximum. The magnetic field strength on the dayside has a dependence on solar zenith angle similar to that found in statistical analyses of the PVO data, although the field is stronger than that measured. The mass loading creates a layer near the planet where the magnetic pressure is replaced with the thermal pressure much like observed. Our studies also raise the question of the role of nightside flow vortices in the formation of the effective obstacle boundary. Overall, our results illustrate that many features seen in the MHD model are consistent with the previously reported observations of the Pioneer Venus Orbiter.