It is a widely held view that the Venusian nightside ionosphere, at least at solar maximum, is formed primarily by the nightward flow of ions which are created on the dayside by photoionization. Suprathermal electrons are believed to be a secondary source. This view is brought into question by in situ measurements made from the Pioneer Venus Orbiter (PVO). Measurements of the electron density, Ne, the solar wind dynamic pressure, Psw, and the total solar EUV flux, VEUV, are employed to examine variations in the average Ne of the nightside ionosphere in response to variations in the solar wind and the solar EUV flux. We examine both the main nightside ionosphere (142--600 km) and the nightside ionosphere at very high altitudes (1400--2500 km), a region known as the ionotail. We find that the orbit to orbit variations in the average Ne are caused primarily by changes in Psw; the magnitude of the response being approximately independent of altitude. Increases in Psw produce more less proportional decreases in the density at all altitudes. This behavior is consistent with the view that the nightward ion flow is controlled by the height of the dayside ionopause, which should be depressed at times of high Psw. The influence of the EUV flux on Ne variations in the ionotail and in the main ionosphere is quite different at the two altitudes, however. The average ionotail density varies directly with the solar EUV flux, while the average density in the main ionosphere varies slightly inversely with the EUV flux. The absence of a strong positive correlation in the lower ionosphere is inconsistent with the nightward flow theory.

Nightward flow should increase with an increase in the solar EUV flux, because the latter increases the dayside density, increases the ionospheric plasma pressure, expands the ionosphere, and creates a larger channel for ion flow across the terminator. This enhanced flow should increase Ne on the nightside, but the reverse is observed in the main ionosphere, at least at solar maximum when the main nightside ionosphere could be measured in situ by PVO. These results suggest that the nightside ionosphere density represents a balance between local ion production by suprathermal electrons and loss by planetary escape. Specific further analysis of existing in situ measurements is recommended to help resolve this question. ¿ American Geophysical Union 1990