INITIAL PVO EVIDENCE OF ELECTRON DEPLETION SIGNATURES DOWNSTREAM OF VENUS

This first analysis of Pioneer Venus Orbiter (PVO) plasma analyzer ele ctron measurements obtained in early 1992 during the PVO entry phase o f the mission indicates the presence downstream from the terminator of a depletion or ''bite out'' of energetic ionosheath electrons similar to that observed on Mariner 10. There is more than one possible expla nation for this energetic electron depletion. If it is due to atmosphe ric scattering, the electrons traveling along draped magnetic lux tube s that thread through the Venus neutral atmosphere would lose energy f rom impact ionization with oxygen. The cross-section for such electron impact ionization of oxygen has a peak near 100 eV, and it remains hi gh above this energy, so atmospheric loss could provide a natural proc ess for electrons at these energies to be selectively removed. In this case, our results are consistent with the Kar et al. (1994) study of PVO atmospheric entry ion mass spectrometer data, which indicates that electron impact plays a significant role in maintaining the nightside ionosphere. Although it is appealing to interpret the energetic elect ron depletion in terms of direct atmospheric scattering, alternatively it could result from strong draping which connects the depletion regi on magnetically to the weak downstream bow shock and thereby reduces t he electron source strength.