Model of Venus ionopause formation
Matney, Mark John
Cloutier, Paul A.
Doctor of Philosophy
A model is presented that simulates the physics of the Venus mantle plasma. A modified magnetohydrodynamic (MHD) fluid picture is assumed where the post-shock solar wind plasma is mass-loaded by photoionizations and other atomic interactions with exospheric neutral atoms. By assuming Newtonian pressure profiles and draped magnetic field geometry in the mantle, the three-dimensional steady-state flow problem is reduced to a one-dimensional calculation along the stagnation (subsolar) flowline. In addition, the validity of the model assumptions and questions about the plasma thermodynamics are addressed. When the resulting model is run using various solar wind conditions, the computed magnetic field features correspond with those measured by the Pioneer Venus spacecraft. The model reproduces the observed region of sharp ion density gradients, known as the ionopause, that separates the mantle plasma from the denser ionospheric plasma below. The straightforward application of the model reproduces the shape and location of the low-altitude ionopause cases well, but for solar wind conditions that correlate with high-altitude ionopauses, the computed ionopauses tend to be lower than those observed. The addition of anomalous heating terms to the model, however, raises the computed ionopause to locations consistent with the medium altitude cases. The inability of the model to adequately describe the high altitude cases may indicate that they are transient events and thus cannot be simulated in steady state. While the source of the anomalous heating is not specified, the presence of hyperthermal ions or plasma-wave interactions are suggested as possible heating mechanisms.
Plasma physics; Astronomy; Astrophysics