Models for the nighttime Venus ionosphere

Possible models for the nighttime Venus ionosphere are developed and compared with observations obtained from Mariner 5. The preferred model assumes the presence of a light reactive ion, such as He+ or H2+, in the topside ionosphere, and it is shown that all features of the observed ionization profile can be accounted for with this single hypothesis. It is suggested that He+ and, or, H2+ can be readily supplied by lateral transport from the dayside. Implications of the model with regard to the abundance of He and H2 are briefly explored, and some consequences for Venus nightglow are discussed.