On the role of minor neutrals in determining the characteristic features of the Venus ionosphere at low altitudes

Abstract

ABSTRACT Electron density profiles obtained from radio occultation experiments reveal significant ionization occurring at altitudes below 120 km in the Venusian ionosphere. The presence of such ionization remains unexplained when considering only major neutrals and the resulting ion chemistry. In this study, we investigate the role of minor neutrals, including NO, O2, C, Ar, H2, and H, in the lower altitudes of the Venusian ionosphere using a one-dimensional photochemical model and Akatsuki radio science measurements. We derive the density of minor neutrals from the profiles of major neutrals. Our analysis demonstrates that NO+ and O$_2^+$ ions exhibit a notable presence below 140 km. When we incorporate the minor neutrals, electron density increases below 115 km but remains unchanged above this altitude. Specifically, the density of NO+ increases below 125 km, with corresponding reductions in O$_2^+$. The increase in NO+ above 115 km can be attributed to the charge-exchange reaction between O$_2^+$ and NO. However, below 115 km, the photoionization of NO results in a net increase in NO+ and electron density. These findings highlight the significant role played by minor neutrals, particularly below 115 km, in shaping the Venusian ionosphere at low altitudes.