Nightside ion composition measurements from the Pioneer Venus Orbiter ion mass spectrometer are compared for solar maximum conditions encountered during the early part of the mission and near solar minimum conditions that prevailed just prior to the end of the mission. A drastic decrease in the O+ ion abundance takes place as the solar activity level approaches solar minimum conditions. This indicates a greatly reduced flux of O+ ion transport across the terminator. At the same time, distinct O+2 layers were observed with peak densities only slightly reduced from solar maximum conditions. The relationship between the solar minimum O+ and O+2 profiles provides the first clear in situ evidence for a nightside ionosphere maintained primarily by electron impact, presumably penetrating solar wind electrons. Observation of excess production of mass 28 ions (over chemical production) provides further evidence of the presence of electron impact ionization. A modest change in He+ densities in the region of the predawn bulge does not have a counterpart in neutral helium. Absent a corresponding change in ionizing electron fluxes, this result indicates that during solar maximum the ionization of nightside helium is primarily due to transport of He+ while electron impact can account for He+ densities as solar minimum is approached. Further, the behavior of hydrogen ions over a solar cycle suggests that impact ionization was significant in producing H+ in 1992, but less than that for He+ ions in that year. ¿ American Geophysical Union 1994