Solar wind-driven plasma fluxes from the Venus ionosphere

Measurements conducted with the ASPERA-4 instrument and the magnetometer of the Venus Express spacecraft show that the kinetic pressure of planetary O+ ion fluxes measured in the Venus wake can be significantly larger than the local magnetic pressure, and as a result, those ions are not solely being driven by magnetic forces but also by the kinetic energy of the solar wind. Beams of planetary O+ ions with those properties have been detected in several orbits of the Venus Express through the Venus wake as the spacecraft traverses by the noon-midnight plane along its near-polar trajectory. The momentum flux of the O+ ions leads to superalfvenic flow conditions. It is suggested that such O(+)ion beams are produced in the vicinity of the magnetic polar regions of the Venus ionosphere where the solar wind erodes the local plasma leading to plasma channels that extend downstream from those regions. The distribution of the number of cases where superalfvenic and subalfvenic conditions are measured along the Venus Express trajectory leads to dominant values when the total kinetic plasma pressure (including that of the solar wind protons) and the magnetic pressure are comparable, thus suggesting a possible equipartition of energy between the plasma and the magnetic field.