On the maintenance of the Venus nightside ionosphere: Electron precipitation and plasma transport

Abstract

Suprathermal integral electron spectra between 5 and 45 eV measured by the Pioneer Venus orbiter RPA are presented for the Venus nightside ionosphere. The observed integral electron flux is relatively constant with time and altitude. The simultaneously measured plasma density is much more variable and not correlated with the electron flux. For a typical electron spectrum the ionization rates and the ion density height profiles for O + and O 2 + are calculated for 10 and 90° magnetic dip angle. The O + and O 2 + ion density height profiles are also calculated for a downward flux of O + ions at 10, 30, and 90° magnetic dip angle. Comparison of the numerical modeling results with median profiles of O + and O 2 + ions measured by the RPA reveals that a downward flux of O + ions of between 1 and 2 × 10 8 cm −2 s −1 satisfactorily reproduces both the O + and O 2 + measured median nightside density profiles. The typical suprathermal electron spectrum produces an O 2 + density profile with peak density approximately half that typically observed and produces an O + profile which is about an order of magnitude too small. From the evidence presented in this paper, from previous measurements of adequate O + transport across the Venus terminator, and from interpretation of ion thermal measurements, we conclude that transport of O + ions from the dayside ionosphere is responsible for most of the ionization rate required to maintain the nightside ionosphere. Variation in the O + transport mechanism is primarily responsible for the large variation of the nightside ionosphere density. Suprathermal electrons provide a relatively constant ionization rate which is of the order of one fourth that of O + transport and which contributes principally to the O 2 + peak.