Pioneer Venus Orbiter contributions to a revised Venus reference ionosphere

Abstract

The original Venus International Reference Atmosphere (VIRA) included an empirical model of the Venusian ionosphere (Bauer et al. 1985) which was based primarily on in situ and radio occultation measurements made by the Pioneer Venus Orbiter (PVO) in 1979 and 1980, a time of very high solar activity. The solar cycle variations of in situ parameters could not be determined because periapsis rose out of the main ionosphere toward the end of 1980 and remained there for the next 11 years. During this period, in situ measurements of the upper nightside ionosphere were made each time periapsis swept through the umbra. Radio occultation measurements continued to provide detailed information on the solar cycle variations of electron density, N e, in the main ionosphere. In situ measurements of the main ionosphere resumed briefly in the Summer and Fall of 1992 as periapsis returned to low altitudes. At that time, the remaining fuel supply was used to delay the inevitable atmospheric entry in order to obtain extensive in situ measurements down to altitudes in the vicinity of 140 km. In this paper we present N e models for solar maximum and solar minimum, and we call attention to what has been learned from the in situ measurements made since the time of the original VIRA model, focusing primarily on the solar cycle variations. These results are offered as a PVO contribution to a planned future revision of the VIRA ionosphere model.