Observations of ionospheric escape on Venus' nightside

A population of low‐energy (0–250 V E/q ions with tailward directed velocity vectors and energies above that for escape from Venus is evident in nightside data from the Ames plasma analyzer on the Pioneer Venus Orbiter spacecraft. Good correlations with solar wind parameters were not obtained for the magnitudes of these ion fluxes, but tendencies for occurrence at times of tailward oriented magnetic fields and for alignment of the ion flows with the magnetic field were found. These tendencies seemed to be enhanced for higher‐energy ions. In a few cases where comparisons were made, the ion fluxes were consistent with simultaneous O + measurements by the neutral mass spectrometer experiment on the spacecraft. The mean flux observed of the escaping nightside ions, averaged over an approximately 10‐week‐long spacecraft nightside season, was < 2×10 6 cm −2 s −1 .