ION DENSITY, TEMPERATURE, AND COMPOSITION OF THE VENUS NIGHTSIDE IONOSPHERE DURING A PERIOD OF MODERATE SOLAR-ACTIVITY - IMPLICATIONS FOR MAINTAINING THE CENTRAL NIGHTSIDE

Ion density, temperature, and the partial densities of the main consti tuents of the Venusian nightside ionosphere during moderate solar acti vity are presented. The data were measured by the retarding potential analyzer of the Pioneer Venus Orbiter during the entry phase of the mi ssion in 1992. The highly variable plasma density is, on average, clea rly reduced relative to that measured at high solar activity. A signif icant dawn-dusk asymmetry, an excess of H+ at dawn and O+ at dusk, occ urs in the postterminator sector. In the central nightside sector, bey ond 150 degrees solar zenith angle, the ionosphere is strongly deplete d. The ion temperature distribution is similar to that measured during high solar activity, but the values are slightly smaller. The measure ments indicate that the ion flux across the terminator, which is the d ominant maintenance source for the nightside ionosphere at high solar activity, decreases with decreasing solar activity. At moderate solar activity we find that plasma transport and particle precipitation cont ribute approximately equally to the ionization of the central sector.