Planetary ionospheres: the venus energetics

Abstract

The Langmuir probe and the Retarding Potential Analyser experiments on the Pioneer Venus Orbiter (PVO) made the first measurements of electron and ion temperatures in the ionosphere of Venus from 1978 to 1992. The observed temperatures were found to be much higher than those estimated before the PVO mission and despite the long Venus night (58 earth days) temperatures did not show much of day to night change. This paper first reviews the PVO results and then discusses the various theoretical models which have attempted to explain these observations. Since the Venus ionosphere is always in a magnetised state, the daytime high temperatures can be explained on the basis of the low thermal conductivity due to the presence of the horizontal magnetic fields. The nighttime temperatures need a small amount of heat input which can be conducted from the dayside. The highly elevated temperatures in the ionosphere region are due to low electron cooling rates resulting from the low electron density in this region.