Particle Precipitation and Odd Nitrogen in the Nighttime Thermosphere of Venus

Emission lines of atomic oxygen at 1304 and 1356 Å, which have been detected by the Pioneer Venus Orbiter (PVO) Ultraviolet Spectrometer, suggest the presence of particle precipitation into the nightside atmosphere of Venus. Fluxes of energetic electrons have also been detected in the Venus umbra by the PVO Retarding Potential analyzer and reported by Knudsen and co-workers; evidence from the PVO Radio Occultation Experiment suggests that the nightside ionosphere is maintained by electron precipitation during periods of low solar activity. The effect of electron precipitation on the odd nitrogen chemistry is discussed and compared to that which occurs in the terrestrial auroral region. The concentration of NO ⁺ is predicted to be enhanced during periods of intense electron precipitation. Data from the PVO Ion Mass Spectrometer, which relate to the presence of particle precipitation, are discussed.