Venus ionopause during solar minimum

During solar minimum, the Venus ionosphere is weak and the solar wind depresses the ionopause to a limiting altitude (Knudsen et al., 1987). With the knowledge gained from the ion composition measurements on Pioneer Venus during solar maximum and during conditions of high solar wind pressure, the authors argue that the typical topside electron density profile at Venus during solar minimum has two distinct regimes; one from about 140 km (the altitude of peak electron density) to 180 km and the other above 180 km. While the former is dominated by O2+ ions which are in photochemical equilibrium, the latter is dominated by O+ ions which are strongly disturbed by the solar wind induced plasma transport. The disturbed ionosphere is formed in the photodynamical regime and has a scale height which is several times smaller than that expected under undisturbed conditions when the ionosphere is in diffusive equilibrium. The small scale height of the disturbed ionosphere is nearly equal to that of the ionizable constituent, atomic oxygen, and is only slightly larger than the chemical equilibrium scale height of the underlying chemical equilibrium region. While the photodynamical ionopause occurs rarely during solar maximum and only when the solar wind pressure is large, we believe that this kind of ionopause is observed much more frequently during solar minimum. The authors find evidence for this in the radio occultation data from Pioneer Venus, Mariner 10 and Venera 9 and 10