  Measurements conducted with the ASPERA-4 instrument and the magnetometer of
the Venus Express spacecraft show that the dynamic 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 being driven by magnetic
forces but 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 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.
