Model studies of the ionosphere of Venus: Ion composition, energetics and dynamics.

Abstract

The upper atmosphere and ionosphere of Venus have been explored by a series of Mariner and Venera spacecrafts, and the Pioneer Venus Orbiter (PVO). Most of our observational data base has been obtained by the PVO, which was placed in orbit around Venus in 1978, and discovered many surprising facts, such as high plasma temperatures on both the day and the nightside, and supersonic horizontal flows. Theoretical models have been important tools in elucidating the physical and chemical processes of the ionosphere. In this thesis, a number of existing models are expanded and new ones developed in order to improve our understanding of the ionosphere of Venus. The hot oxygen and hydrogen models of Nagy et al. (1981), and Cravens et al. (1980) are used to study the hot atom corona around Venus and Mars, and to investigate the O$\sp+$ ion heating by hot oxygen, in conjunction with a multispecies energy model. A photochemical model is used to calculate the electron densities for both solar cycle maximum and minimum daytime conditions; the results of this model are compared with the radio occultation measurements, and are found to be in reasonably good agreement. A multispecies temperature model is developed and used to study the energetics of different ion species. The temperature differences between the ions is found to be small. The O$\sp+$ ion heating mechanism from charge exchange between the O$\sp+$ ions and hot oxygens, suggested by Knudsen (1990), is also studied with the multi-temperature model. A comprehensive MHD model, which solves the coupled continuity, momentum, energy and Maxwell's equations, is presented and used to study the interaction between the MHD and energetics components of the model. The typical high altitude ion velocities, directed toward the nightside, are supersonic, according to the PV measurements. Knudsen et al. (1980) suggested that a shock wave might be formed in the wake to slow the flow as it converged on the antisolar axis. The results from a two-dimensional ionospheric model are presented which solves the coupled continuity, momentum and energy equations, using a second-order form of the "shock-capturing" Godunov scheme, with heat conduction. These results elucidate the importance of these high speed flows and, for the first time, address in a quantitative way, the issue of shock formation in the nightside ionosphere of Venus.