CHEMISTRY OF THE NIGHTSIDE IONOSPHERE OF VENUS

We have constructed a one-dimensional model of the nightside ionosphere of Venus in which it is assumed that the ionization is maintained by day-to-night transport of atomic ions. Downward fluxes of O+, C+ and N+ in the ratios measured on the dayside at high altitudes are imposed at the upper boundary of the model (about 235 km). We discuss the resulting sources and sinks of the molecular ions NO+, CO+, N+, CO2+ and O2+. As the O+ flux is increased, the peak density of O+ increases proportionally and the altitude of the peak decreases. The O2+ peak density is approximately proportional to the square root of the O+ flux and the peak rises as the O+ flux increases. NO+ densities near the peak are relatively unaffected by changes in the O+ flux. If the ionosphere is maintained mostly by transport, the ratio of the peak densities of O+ and O2+ indicates the downward flux of O+ independent of the absolute magnitudes of the densities. The densities of mass-28 ions are, however, still considered to be the most sensitive indicator of the importance of electron precipitation. We examine here the inbound and outbound portions of six early nightside orbits with low periapsis and use data from the Pioneer Venus orbiter ion mass spectrometer, the retarding potential analyzer and the electron temperature probe to determine the relative importance of ion transport and electron precipitation. For most of the orbits, precipitation is inferred to be of low to moderate importance. Only for orbit 65, which was the first nightside orbit published by Taylor et al. [J. geophys. Res. 85, 7765 (1980)] and for the inbound portion of orbit 73 does the ionization structure appear to be greatly affected by electron precipitation.