VENUS BOW SHOCKS AT UNUSUALLY LARGE DISTANCES FROM THE PLANET

Recent analysis of data from the Pioneer Venus Orbiter (PVO) has shown that the bow shock often travels to unusually large distances from th e planet when the solar wind magnetosonic Mach number is near unity. W e suggest that distant bow shocks can be explained as an integral part of the response of the global solar wind/Venus interaction to the ano malous local solar wind conditions that existed during the time of the se observations. The lower than normal plasma beta and magnetosonic Ma ch number are in a parameter regime for which the usual fast-mode bow shock close to the planet may not provide the necessary compression an d deflection of the solar wind. Using MHD simulations we show that, fo r these conditions, the usual fast shock is replaced by a bow shock co nsisting of an intermediate shock near the Sun-Venus line and a fast s hock at large distances from the Sun-Venus line. This composite bow sh ock propagates upstream away from the planet at a low speed and appear s to be approaching a new equilibrium stand-off location at a large di stance from the planet.