Upward ion flow in ionospheric holes on Venus

Abstract

Ion composition measurements made by the orbiter ion mass spectrometer on the Pioneer Venus Orbiter are used to determine the ion flow characteristics within two ionospheric holes on the nightside of Venus. A comparison of the altitude profiles of the observed ion densities with those expected under diffusive equilibrium conditions indicates that the major ions O+, NO+, and O2+ and the minor ions H+ and He+ flow upward, away from Venus, along the axis of the holes. The result is substantiated by a quantitative evaluation of the ion flow speeds appearing in expressions derived from the equations for conservation of mass and momentum of the ions and electrons. The resulting equations for the ion flow speeds are functions of the observed ion and neutral species densities and the ion and electron temperatures. The analysis reveals that all ion species flow upward in the holes because the upward force produced by the plasma pressure gradient exceeds all downward forces. Furthermore, the ion flow speeds are found to increase with altitude as a result of the continual acceleration caused by the net upward force. In the acceleration region, where the observing altitudes were limited to the range extending from the [O+] peak (∼170 km) to 250 km, none of the ion components attain their acoustic speeds, although the light ions come within about a factor of 2. The light ions H+ and He+ attain the greatest speeds because as minor ions they experience the largest polarization electric fields produced in the ion‐electron gas. Although upward ion flow is inferred from the ion composition measurements, the nature of the ion source required to maintain such flow remains a puzzle. For example, the classical diffusion rate of O+ across the magnetic field, into the base of the holes, is about an order of magnitude less than what is needed to feed the upward O+ flux.