Suprathermal electron fluxes in the Venus nightside ionosphere at moderate and high solar activity

Abstract

Suprathermal electron fluxes up to energies of 50 eV measured at moderate solar activity (MSA) during the Pioneer Venus Orbiter entry mission are presented and compared with measurements made during high solar activity (HSA). The median height profiles measured at these two seasons do not significantly differ from each other in the Venus nightside ionosphere. At HSA the fluxes decrease significantly with increasing solar zenith angel (SZA). At MSA, only a small decrease in flux is suggested from individual orbits and a statistical study. Flux changes between different orbits and locations during MSA are usually less than an order of magnitude, a relatively small value compared with the spread 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 smaller. In other cases the suprathermal electrons are approximately twice as energetic as photoelectrons of the dayside, comparable with the characteristic energy of solar wind electrons and assumed to originate there. The results indicate that the source of high thermal plasma densities, which occur especially at HSA, is mainly plasma transport from the dayside ionosphere. The median total plasma density above 200‐km altitude 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.