In December of 1978, the Pioneer Venus Orbiter/Probe (PV) Missions will conduct a comprehensive investigation of the ionosphere of Venus. The purpose of this paper is to bring into focus our very limited current knowledge of the chemistry and physics of the Cytherean ionosphere preparation for the major changes in this understanding that are undoubtedly going to occur as the result of the Pioneer Venus observations.

We have performed theoretical calculations of the energetics of the Cytherean ionosphere and we conclude that the PV measurements will find elevated ion and electron temperatures, resulting primarily from energy fluxes associated in some manner with the solar wind, and acting directly on the ion gas. Ultraviolet dayglow intensities were also calculated and we anticipate that the PV will observe hundreds of kR's of CO2 related emission features such as the CO Cameron bands. Nightside ionosphere calculations were also made assuming the precipitation of energetic electrons as an ionization source, in order to reproduce observed (Venera 9, 10; Mariner 10) nightside electron density profiles. The intensities of some of the resulting emission features are also calculated.