Models of the Nightside Ionosphere of Venus
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The major mechanisms for maintenance of the nightside ionosphere of Venus have been the subject of much debate, although it now appears that transport of ions from the dayside and electron impact are the most important mechanisms at high and low solar activity, respectively. Examination of the high solar activity data from the ion and neutral mass spectrometers on Pioneer Venus has suggested that even at times of high solar activity the relative importance of the two sources varies. We have constructed models of the nightside ionosphere of Venus, which include both electron impact and transport of ions from the dayside. We find that the relative importance of the two sources varies among the different ions, from mass-28 ions, which are produced mainly by electron precipitation, to O+, which is produced almost exclusively by transport from the dayside. We discuss what can be learned from the relative abundances of ions about the major source mechanisms. The most important region of the nightside ionosphere regarding electron precipitation is the chemical equilibrium region, below about 165 km. Since periapsis was below 150 km only rarely during the initial phase of the mission, most of the photochemical equilibrium region, including the ion peak region, remains largely unexplored. We will argue that this region could and should be investigated during the re-entry phase of the Pioneer Venus mission.
Fox, J. L.
(1992). Models of the Nightside Ionosphere of Venus. Eos, 73 (14 - Supplement), 191-192.