SUPRATHERMAL ELECTRON-FLUXES IN THE VENUS NIGHTSIDE IONOSPHERE AT MODERATE AND HIGH SOLAR-ACTIVITY

Suprathermal electron fluxes up to energies of 50 eV measured at moder ate solar activity (MSA) during the Pioneer Venus Orbiter entry missio n are presented and compared with measurements made during high solar activity (HSA). The median height profiles measured at these two seaso ns do not significantly differ from each other in the Venus nightside ionosphere. At HSA the fluxes decrease significantly with increasing s olar zenith angel (SZA). At MSA, only a small decrease in flux is sugg ested from individual orbits and a statistical study. Flux changes bet ween different orbits and locations during MSA are usually less than a n order of magnitude, a relatively small value compared with the sprea d of the thermal plasma density of more than 2 orders of magnitude at altitudes above 200-km altitude. Independent of altitude and SZA, the shape of the spectrum of the suprathermal electrons is similar to the spectrum of the dayside photoelectrons with a characteristic energy of about 7 eV in many cases, but the flux is an order of magnitude small er. In other cases the suprathermal electrons are approximately twice as energetic as photoelectrons of the dayside, comparable with the cha racteristic energy of solar wind electrons and assumed to originate th ere. The results indicate that the source of high thermal plasma densi ties, which occur especially at HSA, is mainly plasma transport from t he dayside ionosphere. The median total plasma density above 200-km al titude in the central nightside sector at MSA is depleted by a factor of 5 compared with that at HSA. Between HSA and MSA, plasma transport is typically reduced by a factor of 6. At MSA, transport and particle precipitation contribute about equally to the nightside ionization.