The Ionospheric Source of the Red and Green Lines of Atomic Oxygen in the Venus Nightglow

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We have modeled the nightside ionosphere of Venus for 11 solar zenith angles between 90° and 165° with a view toward determining the intensities of the atomic oxygen red, green and 2972Å features in the nightglow. The ionospheric source of these emissions is mainly dissociative recombination of O2+, which may proceed via four channels, which produce two O atoms in various combinations of the ground O(3P) state and the electronically excited O(1D) and O(1S) states. These excited states are the precursors to the nightglow features. We have modeled the O2+(v) vibrational distribution for each of the models, and we have used it to determine branching ratios for the DR channels. We have assumed that the ionosphere is produced by transport of atomic ions from the dayside, and that the ratios of the atomic ions are the same as the model values of the upward ion fluxes on the dayside. For each model, we have normalized the downward atomic ion fluxes to the peak O+ densities that were measured by the Pioneer Venus Orbiter Ion Mass Spectrometer. For the non-sunlit models, the O2+ densities peak in the altitude range 152–159km. We have computed the density profiles of O(1D) and O(1S), and the integrated overhead intensities of the transitions for each of the models. We find that the average intensities, weighted by the area of the solar zenith angle bins, for the red, green and 2972Å emissions are 26R, 4.6R, and 0.46R, respectively. Thus the average high solar activity nightside ionosphere cannot produce green line intensities that are as large as the maximum intensity of about 150R as measured from the ground. Our predicted red line intensities are, however, larger than the upper limits determined by ground based and spacecraft measurements. Possibilities for increasing the green line intensities and decreasing the red line intensities are discussed.