Plasma erosion in the Venus nightside ionosphere

Abstract

Pioneer Venus Orbiter (PVO) data from the orbiter electron temperature probe (OETP) measurements show that near the midnight plane the electron density profiles in the upper ionosphere are different from those obtained throughout most of the nightside hemisphere. The density profiles near the midnight plane have a near constant density plateau that extends from very low (∼200 km) altitudes located near the PVO trajectory periapsis up to the ionopause crossing that may reach high (∼2300 km) altitudes. Such profiles are interpreted in terms of ionospheric plasma that has been dragged by the solar wind that streams over the nightside ionopause and also within the polar plasma channels. The latter features are produced by the solar wind that carves out the upper ionospheric plasma near the magnetic polar regions and can account for the presence of the ionospheric holes detected in the nightside hemisphere. The solar wind-directed displacement of the ionospheric plasma in the vicinity of the plasma channels results from the transfer of the momentum of the solar wind that moves within them. Data compatible with the view that the PVO moved within a plasma channel before entering the ionosphere (ionosheath–channel–ionosphere transit) are also examined. It is shown that the orientation and intensity of the magnetic field within the channels is similar to the values observed in the ionospheric holes and it is argued that these conditions are set by the magnetic field that accumulates within the channels in response to the thermal pressure made by the solar wind on the polar upper ionosphere. The temperature of the hot electron component in the holes (channels) is much smaller than that in the ionosheath flow because a large fraction of the latter thermal energy density may be replaced by the magnetic energy density of the (∼30 nT) magnetic field that accumulates within the channels and that is measured within the holes.