Rarefied plasma fluxes within the polar expansion fan of the Venus ionosheath

Abstract

Abstract Measurements obtained with the PVO plasma instrument show strong changes in the particle flux intensity at the intermediate transition of the Venus ionosheath. In particular, the strong plasma fluxes present in the outer ionosheath sharply decrease to low values across this transition and are not observed in the inner ionosheath. These conditions are most frequently seen near the regions where the draped interplanetary magnetic field lines slip over the planet to enter the wake (magnetic polar regions) and imply the presence of an expansion fan that extends in the downstream direction of the solar wind. Low magnetic field intensities are also observed within the expansion fan with a sharp change at the intermediate transition. Similar characteristics can be obtained from data obtained during the Mariner 5 flyby across the Venus wake. In this case there is evidence of a strong decrease of the solar wind flow speed across the intermediate transition downstream from the terminator. The magnetic field data obtained in that pass further indicates that this event was detected as the spacecraft moved behind a magnetic polar region and that as a result the solar wind becomes strongly decelerated over those regions. Thus, in addition to having low densities and magnetic field intensities the plasma within the expansion fan decelerates and exhibits weak particle fluxes.