A Three Dimensional Model of the Plasma Flow and Magnetic Fields in the Dayside Ionosphere of Venus

摘要

Today, the earth's instrinsic magnetic field prevents the streaming solar wind plasma from directly interacting with the terrestrial atmosphere. Periodically the geomagnetic field reverses direction, and during the transition period, the earth's magnetic barrier is thought to disappear. In the past, studies about the atmospheric-solar wind interaction dynamics were hindered by a scarcity of observational data. However, since December 1978, the Pioneer-Venus Orbiter has been providing daily observations of the atmospheric dynamics produced by the direct solar wind interaction with the atmosphere of Venus (the only planet known to lack an intrinsic magnetic field). This thesis develops the first three dimensional magnetohydrodynamic (MHD) theory of this interaction. Within the ionosphere of Venus analytic solutions to the MHD equations are possible because of a favorable geometry between the induced ionospheric magnetic fields and the ionospheric plasma motions. It is shown that variations in the solar wind speed and interplanetary magnetic vector direction cause variations in the dayside ionospheric plasma flows and the induced magnetic field configuration, and that these changes can account for the variety of magnetic structures observed by Pioneer-Venus. Portions of the Venus ionosphere are shown to be susceptible to the Kelvin-Helmholtz shear instability. The unusual shape of the computed region of stability is shown to be an important key to understanding the highly variable Pioneer-Venus observations. Model calculations are compared to observations for a number of selected orbits, and the model is shown to match the observations in fine detail.