Friction layer along the sides of the plasma channels in the Venus nightside ionosphere

Electron density profiles of the Venus nightside ionosphere measured with the Pioneer Venus Orbiter (PVO) reveal cases in which there is a plateau signature that extends across the upper ionosphere. Such cases are not seen throughout the nightside hemisphere but occur mostly near the midnight plane. The distribution in the local solar time (LST) position of passes in which this peculiarity is seen was derived from a study of 40 PV orbits that scanned the nightside ionosphere in the first and third season of that spacecraft operation. The dominant contribution of passes with a density plateau is slightly shifted in LST from the midnight plane towards the dawn terminator and thus suggest a displacement in the same direction as that of the distribution of ionospheric holes reported from the early PVO observations. The peak of the distribution occurs in the dawn side vicinity of the midnight plane indicating a displacement that is slightly smaller than that of the ionospheric holes. It is argued that density profiles with a plateau signature represent passes through a friction layer that is formed along the sides of the plasma channels. This result is viewed as implying that the ionospheric material located at the sides of the plasma channels, which provide a useful interpretation of the ionospheric holes, is strongly eroded by the solar wind that flows within the channels. Momentum transport from the solar wind to the ionospheric plasma is assumed to be responsible for eroding the sides of the plasma channels and thus producing density profiles with a plateau signature.