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 evi dent in nightside data from the Ames plasma analyzer on the Pioneer Ve nus Orbiter spacecraft. Good correlations with solar wind parameters w ere not obtained for the magnitudes of these ion fluxes, but tendencie s 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 case s where comparisons were made, the ion fluxes were consistent with sim ultaneous O+ measurements by the neutral mass spectrometer experiment on the spacecraft. The mean flux observed of the escaping nightside io ns, averaged over an approximately 10-week-long spacecraft nightside s eason, was < 2 x 10(6) cm(-2) s(-1).