EVIDENCE FOR DAY-TO-NIGHT ION-TRANSPORT AT LOW SOLAR-ACTIVITY IN THE VENUS PRE-DAWN IONOSPHERE

Periapsis of the Pioneer Venus spacecraft dropped below 180 km on Augu st 28, 1992 near midnight, and 42 orbits of low altitude data at moder ately low solar activity in the pre-dawn sector were obtained before c ontact was lost to the spacecraft in October, 1992. Through a combinat ion of analysis of data from the PV orbiter ion mass spectrometer (OIM S) and modeling, we consider here what can be learned about the relati ve importance of plasma transport from the dayside and electron precip itation in maintaining the nightside ionosphere during the re-entry pe riod. In particular, we examine here the atomic ion density profiles. We compute the average peak density of O+ as a function of solar senit h angle and determine what fluxes of atomic ions or precipitating elec trons would be necessary to produce those values. We then compare mode l calculations of the ion densities to those observed during the re-en try period. We find that the low solar activity nightside ionosphere s hows evidence of significant day-to-night plasma transport.