Ds. Intrilligator et al., INITIAL PVO EVIDENCE OF ELECTRON DEPLETION SIGNATURES DOWNSTREAM OF VENUS, Geophysical research letters, 20(23), 1993, pp. 2779-2782
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.