Development of model of formation of Venusian nighttime ionosphere for period of low solar activity

A 2-D model of the formation of the nighttime ionosphere of Venus for a period of low solar activity which takes into account the influence of two probable sources of its formation (ionization by epithermal electrons and horizontal transport of ion from the daytime side) is tested on the basis of radio occultation data from the Venera 9, 10, 15, 16 spacecraft. Comparison of experimental and computed data supports the validity of the first of these sources; the second, together with photoionization, makes a contribution only in the region adjacent to the planetary terminator and causes a regular decrease in electron concentration at the maximum of the main layer of the ionosphere with an increase in solar zenith angle in the range of 90 to 110 degs. Although the cited data indicate a predominant influence of epithermal electrons during formation of the nighttime ionosphere, this mechanism does not explain the existence of a lower ionization maximum in the nighttime ionosphere. Since the frequency of appearance of this maximum is different in different years, it is postulated that it is formed under the influence of an unknown source whose intensity varies with the solar activity phase.