Evidence of upward H⁺ flow in the Venus dayside ionosphere

Abstract

At Venus H⁺ is expected to be under diffusive equilibrium above about 400 km, but it is difficult to verify this under normal circumstances, because solar wind interaction terminates the subsolar ions around 400 km. However, under conditions of extremely low solar wind pressure, the terminal height, (called the ionopause) can move up to very high altitudes. Such events are rare, but after searching through the Pioneer Venus data, we have been able to identify some orbits with ionopause altitudes above 1000 km. Altitude distributions of O⁺ and H⁺, measured by the ion mass spectrometer experiment, have been studied for these orbits. It is found that while O⁺ distribution is consistent with diffusive equilibrium, H⁺ shows large departures providing evidence of upward flow of H⁺. Approximate calculations indicate that the flow is subsonic.