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 missio n, the Orbiter Ion Mass Spectrometer measurements found all ion specie s, in the midnight-dusk sector, reduced in concentration relative to t hat observed at solar maximum. Molecular ion species comprised a great er part of the total ion concentration as O+ and H+ had the greatest d epletions. The nightside ionospheric states were strikingly similar to the isolated solar maximum ''disappearing'' ionospheres. Both are ver y dynamic states characterized by a rapidly drifting plasma and 30-100 eV superthermal O+ ions.