Ion density, temperature, and composition of the venus nightside ionosphere during a period of moderate solar activity - implications for maintining the central nightside

Ion density, temperature, and the partial densities of the main constituents of the Venusian nightside ionosphere during moderate solar activity are presented. The data were measured by the retarding potential analyzer of the Pioneer Venus Orbiter during the entry phase of the mission in 1992. The highly variable plasma density is, on average, clearly reduced relative to that measured at high solar activity. A significant dawn-dusk asymmetry, an excess of H+ at dawn and O+ at dusk, occurs in the postterminator sector. In the central nightside sector, beyond 150 deg solar zenith angle, the ionosphere is strongly depleted. The ion temperature distribution is similar to that measured during high solar activity, but the values are slightly smaller. The measurements indicate that the ion flux across the terminator, which is the dominant maintenance source for the nightside ionosphere at high solar activity, decreases with decreasing solar activity. At moderate solar activity we find that pl asma transport and particle precipitation contribute approximately equally to the ionization of the central sector.