No abstract

A critical analysis of the current interpretations of the antisolar motion of the Venus ionosphere is presented. It is argued that the pressure gradient forces present across the terminator are not sufficient to produce the observed acceleration of the plasma. A balance condition between the height‐integrated momentum flux of the ionospheric flow and the height‐integrated deficiency of momentum flux of the shocked solar wind in the vicinity of the terminator is formulated in terms of measurable quantities. It is found that the observed 2 to 4‐km/s flow velocities of the upper ionosphere can be readily inferred from such a relation. This result is consistent with the concept that an efficient transport of momentum takes place across the ionopause, and that the bulk of the kinetic energy required to accelerate the Venus ionosphere is derived from the shocked solar wind.

[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.

Abstract. The geometry of the Venus nightside ionosphere is reexamined on the basis of plasma and magnetic field data obtained with the Pioneer Venus Orbiter (PVO) and the Mariner 5 spacecraft, There is evidence indicating that the shocked solar wind that streams over the magnetic polar regions of the Venus ionosphere experiences a sharp expansion and also a strong deceleration to speeds much smaller than those present in the outer ionosheath. It is argued that the deficiency of solar wind momentum that is inferred from the PVO and Mariner 5 measurements along the flanks of the Venus ionosheath may be responsible for the erosion of the polar upper ionosphere in plasma channels that extend downstream from the magnetic polar regions. The erosion of the Venus ionosphere is mostly produced within those channels and represents the main source of material delivered to the Venus wake. The configuration that is proposed accounts for the PVO observation of plasma clouds and ionospheric holes in the nightside hemisphere and leads to a mass loss smaller than that expected from a global erosion. The formation and evolution of the ionospheric polar channels should also be important in the solar wind interaction and erosion of the Mars ionosphere.

We report on the existence of a large‐scale ion flow vortex, a curled tailward flow of solar wind H+ (SW H+), and ionospheric O+ in the Venus plasma tail. The vortex commences at dusk (−Y), driven by a transverse (to the solar wind) aberration flow component. Dusk magnetosheath and ionospheric ions move westward across the nightside into the dawn sector, from where the tailward and lateral flow merges into a tailward‐moving vortex. A fluid analysis of the SW H+ energy and momentum (E&M) transfer to O+ at the terminator, shows that E&M balance (efficiency ≈1) is achieved in the altitude range of 1200–600 km. Below 600 km a westward O+ flow, moving along the direction of the atmospheric superrotation, dominates. Conversely, SW H+ dominates the high‐altitude vortex. The Venus large‐scale tail vortex is hardly unique. Other gaseous celestial objects (comets) orbiting the Sun may develop similar tail vortices.