ION MEASUREMENTS DURING PIONEER VENUS REENTRY - IMPLICATIONS FOR SOLAR-CYCLE VARIATION OF ION COMPOSITION AND DYNAMICS

During the final, low solar activity phase of the Pioneer Venus mission, the Orbiter Ion Mass Spectrometer measurements found all ion species, in the midnight-dusk sector, reduced in concentration relative to that observed at solar maximum. Molecular ion species comprised a greater part of the total ion concentration as O+ and H+ had the greatest depletions. The nightside ionospheric states were strikingly similar to the isolated solar maximum ''disappearing'' ionospheres. Both are very dynamic states characterized by a rapidly drifting plasma and 30-100 eV superthermal O+ ions.