Morphology of O⁺⁺ ions and their maintenance in the nightside Venus ionosphere

Abstract

In this paper, measurements made by the Pioneer Venus orbiter ion mass spectrometer are used to study the temporal and spatial distribution of O⁺⁺ ion concentrations in the Venus ionosphere. The diurnal analysis shows an enhancement of O⁺⁺ around midnight. On the nightside the O⁺⁺ densities are highly variable but show a positive correlation with variations in the O⁺ densities. Model calculations of O⁺⁺ concentrations in the nightside are made assuming that impact ionization by precipitating electrons and transport of ions from the dayside were the sources for O⁺⁺ ions. We find that in situ ionization by precipitating electrons contributes very little, whereas a downward O⁺⁺ flux in the range of 10⁵ to 10⁶ cm^-2 s^-1 can account for the measured values of O⁺⁺ densities near midnight. The high abundance of O⁺⁺ near midnight is related to the special characteristics of multiply charged ions. Because of the double charge, thermal diffusion and the polarization electric field are particularly found to be signifiicant factors in the momentum equations for O⁺⁺, and therefore the downward velocity for O⁺⁺ is small compared with that of other ions. High O⁺⁺ densities are observed to occur under low solar wind dynamic pressure conditions, consistent with the transterminator transport mechanism as the source of O⁺⁺ ions in the antisolar region.