ION DENSITY, TEMPERATURE, AND COMPOSITION OF THE VENUS NIGHTSIDE IONOSPHERE DURING A PERIOD OF MODERATE SOLAR-ACTIVITY - IMPLICATIONS FOR MAINTAINING THE CENTRAL NIGHTSIDE
K. Spenner et al., ION DENSITY, TEMPERATURE, AND COMPOSITION OF THE VENUS NIGHTSIDE IONOSPHERE DURING A PERIOD OF MODERATE SOLAR-ACTIVITY - IMPLICATIONS FOR MAINTAINING THE CENTRAL NIGHTSIDE, J GEO R-S P, 100(A8), 1995, pp. 14499-14506
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.