EVIDENCE OF ELECTRON-IMPACT IONIZATION ON THE NIGHTSIDE OF VENUS FROMPIONEER VENUS ORBITER ION MASS-SPECTROMETER MEASUREMENTS NEAR SOLAR MINIMUM

Nightside ion composition measurements from the Pioneer Venus Orbiter ion mass spectrometer are compared for solar maximum conditions encoun tered during the early part of the mission and near solar minimum cond itions that prevailed just prior to the end of the mission. A drastic decrease in the O+ ion abundance takes place as the solar activity lev el approaches solar minimum conditions. This indicates a greatly reduc ed flux of O+ ion transport across the terminator. At the same time, d istinct O2+ layers were observed with peak densities only slightly red uced from solar maximum conditions. The relationship between the solar minimum O+ and O2+ profiles provides die first clear in situ evidence for a nightside ionosphere maintained primarily by electron impact, p resumably penetrating solar wind electrons. Observation of excess prod uction of mass 28 ions (over chemical production) provides further evi dence of the presence of electron impact ionization. A modest change i n He+ densities in the region of the predawn bulge does not have a cou nterpart in neutral helium. Absent a corresponding change in ionizing electron fluxes, this result indicates that during solar maximum the i onization of nightside helium is primarily due to transport of He+ whi le electron impact can account for He+ densities as solar minimum is a pproached. Further, the behavior of hydrogen ions over a solar cycle s uggests that impact ionization was significant in producing H+ in 1992 , but less than that for He+ ions in that year.