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 ch anging location of the Venus bow shock with solar cycle and provided a detailed picture of the magnetic field pileup in the dayside magnetos heath. Nevertheless, the reason for the increase of the terminator sho ck position to the observed distances has evaded our understanding, an d the ''magnetic barrier'' region has been studied primarily by compar isons with gasdynamic models due to the difficulty of using more sophi sticated 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 photoionizatio n of the atomic oxygen upper atmosphere, can reproduce some of the bas ic features of the dayside magnetic field observed on PVO. The ideal M I-ID model uses a conducting sphere to represent the basic Venus ionos pheric obstacle to the solar wind flow. We adopt the viewpoint that du ring solar maximum, a conducting obstacle with oxygen mass loading is appropriate, while a no-mass loading case is a good first approximatio n to the solar minimum situation. The MHD simulations are found to giv e a realistic picture of both the shape of the bow shock and its obser ved elliptical cross section at the terminator. The introduction of th e oxygen mass loading moves the shock position to that observed at sol ar maximum. The magnetic field strength on the dayside has a dependenc e on solar zenith angle similar to that found in statistical analyses of the PVO data, although the field is stronger than that measured. Th e mass loading creates a layer near the planet where the magnetic pres sure is replaced with the thermal pressure much like observed. Our stu dies also raise the question of the role of nightside flow vortices in the formation of the effective obstacle boundary. Overall, our result s illustrate that many features seen in the MHD model are consistent w ith the previously reported observations of the Pioneer Venus Orbiter.