Plasma channels and electron density profiles near the midnight plane in the Venus nightside ionosphere

Abstract

[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.

Nightside ionosphere of Mars studied with local electron densities: A general overview and electron density depressions

“…At the same time, plasma is removed from the region with either ion subsidence to the lower ionosphere, or ion loss to the magnetotail through electromagnetic processes [Brace et al, 1982]. Pérez-de-Tejada [2004] concluded that the direction of the magnetic field with respect to the ecliptic plane can account for the ionospheric density holes at Venus.…”

Comparative outline of the region of interaction of the solar wind with the Ionosphere of Venus and Mars

“…While evidence of the MPB is now available at both planets it is still necessary to learn whether the sudden decrease of the solar wind proton flux that has been measured at the MPB in the Mars plasma environment is also applicable at Venus since peculiar conditions have been inferred for the solar wind that streams along the flanks of the ionosheath (Pe´rez-de-Tejada, 1999, 2005. Most notable is the existence of a velocity boundary layer in which the magnetic field intensity takes depressed values instead of the enhancement expected within the magnetic pileup region (Bridge et al, 1967;Pe´rez-de-Tejada, 2004).…”

“…Since the interplanetary magnetic field does not strongly accumulate around those regions there should be a direct plasma-plasma interaction between the solar wind and the ionospheric material. Under such circumstances solar wind momentum transport could be responsible at Venus for producing the plasma channels that can account for the observation of ionospheric holes in the nightside hemisphere as well as a source for the trans-terminator flow (Pe´rez-de-Tejada, 1986a, 2004. The conditions at Mars may be significantly more complicated, however, since the solar wind proton and electron components may be replaced by other populations at different altitudes.…”

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

“…Under such circumstances the PVO only has access to the ionosphere at very low altitudes (near periapsis) and remains within a plasma channel as the spacecraft approaches or moves away from the low altitude ionosphere. Support for this view is that in an orbit suitable for this condition the magnetic field intensity and the magnetic field direction measured along the section corresponding to the plasma channel are nearly the same as those usually encountered within the ionospheric holes (Pérez-de-Tejada, 2004a).…”