Solar wind interaction with the ionosphere of Venus inferred from radio scintillation measurements

Abstract

This paper presents the first S‐band (2.3 GHz) radio scintillations observed in the ionosphere of Venus and discovered when the Pioneer Venus Orbiter spacecraft traversed the ionosphere of Venus. In situ plasma measurements as well as propagation calculations confirm that the scintillations are caused by electron density irregularities in the topside ionosphere of Venus below the ionopause. While these topside plasma irregularities have not been studied before, simultaneous magnetic field measurements presented here reveal that they are associated with the penetration of large‐scale magnetic fields in the ionosphere. Previous studies based on extensive magnetic field measurements have shown that the presence of large‐scale magnetic fields occurs in the subsolar region when the solar wind dynamic pressure exceeds the ionospheric plasma pressure. As with the large‐scale magnetic fields, the disturbed plasma and resulting scintillations are therefore a manifestation of high‐dynamic solar wind interaction with the ionosphere. Since the scintillations only occur in the subsolar region of Venus, the global morphology of ionospheric scintillations at Venus is different from that of the terrestrial ionosphere, where scintillations are observed in both polar and equatorial regions, with peaks occurring during nighttime. This difference apparently stems from the fact that Venus is not a magnetic planet. We also demonstrate that the disturbed plasma produced by the high‐dynamic solar wind interaction can be remotely sensed by scintillations during radio occultation measurements, that is, when the spacecraft is outside the ionosphere. Such observations can therefore provide important information on the high‐dynamic solar wind interaction of nonmagnetic planets such as Venus and Mars when in situ measurements are not available. Venus is of particular interest at solar cycle minimum, when high‐dynamic conditions may be expected to be more prevalent.