POLAR EXPANSION FAN IN THE VENUS IONOSHEATH

Magnetic field data of the Pioneer Venus Orbiter (PVO) are used to exa mine the configuration of the intermediate transition around the Venus wake. The intermediate transition is a plasma boundary characterized by a sudden decrease of the magnetic field intensity to low values in the inner ionosheath and a strong rotation of the magnetic field orien tation to a direction closer to the Sun-Venus axis. Measurements show that the intermediate transition is not seen across the entire Venus w ake but in an expansion fan that is formed around the solar wind-induc ed magnetic polar regions of the Venus ionosheath. The magnetic field geometry within the expansion fan is produced by ionosheath magnetic f ield lines that are draped around those regions and that are then conv ected by the shocked solar wind. Examples are shown where the intermed iate transition is seen at positions different from those of ionospher ic plasma clouds. Across both features the magnetic field may also exh ibit a different rotation including changes in the polarity of its com ponent along the Sun-Venus axis, A schematic of the magnetic field geo metry within and in the vicinity of the expansion fan is presented. It is argued that the expansion fan flares away from the Venus tail and that the local orientation of the magnetic field is different from tha t observed around the Venus dayside ionosphere.