Transport of ionospheric O+ ions across the Venus terminator and implications

Vector velocities of O+ ions in the Venus ionosphere are reported for the dawn and dusk terminator regions. The velocity vectors are generally directed antisunward and radially inward toward the planet with magnitudes ranging from approximately 1 to 8 km/s. The velocity generally increases with altitude. The velocity may increase to still larger values just below the ionopause, but experimental limitations prevent measuring such an increase. The Alfven and ion Mach numbers are generally greater than 1. The estimated O+ flux across the terminator is equal within a factor of 2 to the total ion recombination rate on the nightside and is evidently a factor of 10 larger than that which could be exiting down the planet wake. The higher velocity flow near the ionopause is hypothesized to converge on the antisolar axis and, after thermalization, to descend into the ionosphere. The kinetic energy and O+ flux in this high-velocity stream are sufficient to maintain, respectively, the high ion temperature and peak ion concentration, respectively, measured at solar zenith angles in excess of 150. The nightside ionospheric variability is hypothesized to result from temporal and spatial variations in the ionospheric terminator and wake flow fields. Ion transport contributes substantially and possibly predominantly to the maintenance of the nightside ionosphere.