Plasma channels and electron density profiles near the midnight plane in the Venus nightside ionosphere -: art. no. A04106

[1] Pioneer Venus Orbiter (PVO) data with information on the electron densities in the Venus nightside ionosphere are examined in a study of the plasma channels that extend downstream from the magnetic polar regions. The plasma channels are produced by the solar wind that erodes the polar upper ionosphere and provide a useful interpretation of the ionospheric holes as regions of depleted plasma density that are observed in the nightside hemisphere. The plasma channels can also account for the observed distribution of ionospheric holes in the nightside hemisphere as a function of the solar wind dynamic pressure. Wide plasma channels that result from the enhanced erosion of the polar upper ionosphere under high dynamic pressures lead to the ionospheric holes that are observed far from the midnight plane. Data obtained from the PVO show that in addition to the ionospheric holes there are orbits traced near the midnight plane with measurements that are also related to the plasma channels. In some cases the electron density does not change significantly through the upper ionosphere but exhibits a density plateau that extends from a high-altitude nightside ionopause crossing to a low-altitude location where a sharp change in the density is observed near periapsis. In addition, there are PVO passes in which the nightside ionopause detected near the midnight plane is located at very low altitudes, implying that the PVO may have remained within the plasma channels as it approached or moved away from periapsis. Transit through the bottom of the plasma channels can account for the low-altitude ionopause crossings seen in these orbits and also for the sharp change at the low-altitude edge of the density plateau measured in orbits that probed near the midnight plane. Unlike PVO passes in which ionospheric holes are detected near the midnight plane, the magnetic field measured in the solar wind during these latter orbits is not oriented near the ecliptic plane, and thus the magnetic polar regions in the Venus ionosphere and the plasma channels that extend behind them are not encountered along the near polar PVO trajectory. In such cases the spacecraft may not move across the plasma channels but remain along their sides as it travels through the nightside ionosphere. The orientation of the magnetic field with respect to the ecliptic plane can thus account for the observation of ionospheric holes or a density plateau in profiles traced near the midnight plane.