Thesis Committee

William Lotko, Ph. D.

Simon G. Shepherd, Ph. D.

John G. Lyon, Ph. D.

Michael Wiltberger, Ph. D.

Abstract

The forecast capabilities of global magnetohydrodynamics (MHD) simulations of geospace are sensitive to the particular specification of low-altitude (inner) boundary conditions. The low-altitude boundary conditions imposed in all global simulations (at least six different models are in active use around the world) are artificial in varying degrees. Consequently, they introduce nonphysical artifacts in the MHD solution. The principle objectives of this thesis are to improve the low-altitude boundary conditions in global magnetospheric MHD models in two ways: 1) by developing and employing Poynting flux-conserving boundary conditions and 2) by including the effects of field-aligned potential drops in the magnetosphere-ionosphere interaction in the boundary specification. The proposed boundary conditions have been implemented in the Lyon-Fedder-Mobarry (LFM) global simulation model. LFM simulation results are diagnosed to qualify the resulting improvements in the solution.

The results presented in the dissertation illustrate the nonphysical artifacts introduced near the low-altitude boundary by the currently implemented LFM boundary conditions. It is shown that these artifacts are largely eliminated by flux-conserving boundary conditions, which conserve low-frequency (essentially DC) Poynting flux flowing along magnetic field lines. The field-aligned DC Poynting flux just above the boundary is also shown to be very nearly equal to the ionospheric Joule heating, as it should be if electromagnetic energy is conserved.

The effects of field-aligned potential drops attributed to anomalous resistive layers that form at low altitude, in the “gap region" between the inner simulation boundary and the ionosphere, have also been included in the effective boundary condition. The model produces much larger potential drops in regions of upward field-aligned current, which are most prevalent on the dusk side, in contrast with those that occur in downward field-aligned currents that are more prevalent on the dawn side. The net effect of including field-aligned potential drops is a dawn-dusk asymmetry in the state variables for both the magnetosphere and ionosphere.

From these developments it is concluded that implementation of Poynting flux-conserving boundary conditions, with allowance for the formation of field-aligned potential drops, not only improves the fidelity of the simulation near the low-altitude boundary, but these boundary conditions also add novel physics that are consistent with ground-based and satellite observations of various state variables. Thus, significant improvements to the global simulations have been achieved.