A three-dimensional MHD study of solar wind mass loading processes at Venus: Effects of photoionization, electron impact ionization, and charge exchange

Ionization of hot and cold planetary oxygen ions caused by solar radiation, electron impact, and charge exchange with the salar wind protons lead to a heavy plasma population near Venus which is accelerated and incorporated into the solar wind plasma. Ionization of hot and cold hydrogen also enhances the solar wind density and reduces its momentum. Charge exchange of the solar wind plasma with hot and cold planetary hydrogen removes a part of this heavy ion population. We include these processes in a three-dimensional magnetohydrodynamic (MHD) model of the solar wind interaction with Venus and analyze their impact on the magnetic and flow field geometry around Venus. We find that photoionization is the most important of these processes if the change in the plasma composition is taken into account. The ionization processes lead to a strong deceleration of the flow around the planet, which in turn results in changes of the magnetic field. Because of the increasing mean mass and density of the plasma near the ionopause velocity gradients develop in the near terminator region, which lead to a local increase of the magnetic field. The inclusion of mass loading in our model moves the bow shock position outward, close to the location observed by the Pioneer Venus Orbiter magnetometer at solar maximum.