Magnetohydrodynamics Modeling of Space Plasmas with Pressure Anisotropy.

Abstract

The present generation of global 3D magnetohydrodynamic (MHD) simulations of the Sun-Earth environment is based on the assumption that the plasma pressure is isotropic. This assumption, however, is an inadequate description of space plasmas, such as plasmas in the Earth’s magnetosheath and inner magnetosphere, as well as in the solar corona, where strong magnetic fields give rise to highly anisotropic plasma pressures. Specifically, particle collisions are not frequent enough to balance the particle motions along and perpendicular to the magnetic field, thus the corresponding parallel and perpendicular pressure components are different.

This dissertation research, therefore focuses on extending the University of Michi- gan MHD space physics code BATS-R-US to account for pressure anisotropy. The analytical model is developed by studying the formulation of anisotropic MHD under both classical and semirelativistic approximations, in particular, deriving the dis- persion relation and characteristic wave speeds for semirelativistic anisotropic MHD. The software implementation of the new model, Anisotropic BATS-R-US, is verified through numerical tests.

Several applications of Anisotropic BATS-R-US are considered in this work. The first application is to simulate the quiet time terrestrial magnetosphere and validate the results with satellite measurements. Pressure anisotropy is found to widen the magnetosheath, enhance the nightside plasma pressure, and reduce the flow speed in the magnetotail. In the second application, Anisotropic BATS-R-US is coupled with two ring current models, respectively, to conduct global magnetospheric simulations during geomagnetic disturbed times. The simulation results indicate the importance of pressure anisotropy in controlling the nightside magnetic field topology. Finally, Anisotropic BATS-R-US is applied to simulate the solar corona and heliosphere, in which pressure anisotropy results in faster solar wind speeds close to the Sun. This application has the potential to capture the anisotropic heating mechanism that has not been addressed by isotropic MHD models.