Plasma clouds above the ionopause of Venus and their implications

Early Pioneer Venus orbiter measurements by the Electron Temperature Probe (OETP) have revealed wavelike structures at the ionopause and clouds of plasma above the ionopause, features which may represent ionospheric plasma at different stages in its removal by solar wind-ionosphere interaction processes. Continuing operation of the orbiter through three Venus years has now provided enough additional examples of these features to permit their morphologies to be examined in some detail. The global distribution of the clouds suggests that they originate at the dayside ionopause as wavelike structures which may become detached and swept downstream in the ionosheath flow. Alternatively the clouds may actually be attached streamers analogous to cometary structure. Estimates of the total ion escape rate from Venus by this process yields values up to 7 × 10²⁶ ions s^?1, based on their measured transit times, their probability of occurrence, their statistical distribution and their average electron density. Preliminary analysis shows that such an excape flux could be supplied by the upward diffusion limited flow of 0⁺ from the entire dayside ionosphere. Observed distortions of dayside ionosphere height profiles suggest that such flows may be present much of the time. If such an escape flux were to continue over the entire lifetime of Venus, the effects upon the evolution of its primitive atmosphere may have been significant.