Pressure anisotropy in global magnetospheric simulations: A magnetohydrodynamics model

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95331/1/jgra22024.pd

Magnetohydrodynamics Modeling of Space Plasmas with Pressure Anisotropy.

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.PHDAtmos, Oceanic & Space Science & Scientific ComputingUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/97946/1/xingm_1.pd

Gas Kinetic Schemes for Solving the Magnetohydrodynamic Equations with Pressure Anisotropy

In many astrophysical plasmas, the Coulomb collision is insufficient to maintain an isotropic temperature, and the system is driven to the anisotropic regime. In this case, magnetohydrodynamic (MHD) models with anisotropic pressure are needed to describe such a plasma system. To solve the anisotropic MHD equation numerically, we develop a robust Gas-Kinetic flux scheme for non-linear MHD flows. Using anisotropic velocity distribution functions, the numerical flux functions are derived for updating the macroscopic plasma variables. The schemes is suitable for finite-volume solvers which utilize a conservative form of the mass, momentum and total energy equations, and can be easily applied to multi-fluid problems and extended to more generalized double polytropic plasma systems. Test results show that the numerical scheme is very robust and performs well for both linear wave and non-linear MHD problems

Real‐Time SWMF at CCMC: Assessing the Dst Output From Continuous Operational Simulations

The ground‐based magnetometer index of Dst is a commonly used measure of near‐Earth current systems, in particular the storm time inner magnetospheric current systems. The ability of a large‐scale, physics‐based model to reproduce, or even predict, this index is therefore a tangible measure of the overall validity of the code for space weather research and space weather operational usage. Experimental real‐time simulations of the Space Weather Modeling Framework (SWMF) are conducted at the Community Coordinated Modeling Center (CCMC). Presently, two configurations of the SWMF are running in real time at CCMC, both focusing on the geospace modules, using the Block Adaptive Tree Solar wind‐type Roe Upwind Solver magnetohydrodynamic model, the Ridley Ionosphere Model, and with and without the Rice Convection Model. While both have been running for several years, nearly continuous results are available since April 2015. A 27‐month interval through July 2017 is used for a quantitative assessment of Dst from the model output compared against the Kyoto real‐time Dst. Quantitative measures are presented to assess the goodness of fit including contingency tables and a receiver operating characteristic curve. It is shown that the SWMF run with the inner magnetosphere model is much better at reproducing storm time values, with a correlation coefficient of 0.69, a prediction efficiency of 0.41, and Heidke skill score of 0.57 (for a −50‐nT threshold). A comparison of real‐time runs with and without the inner magnetospheric drift physics model reveals that nearly all of the storm time Dst signature is from current systems related to kinetic processes on closed magnetic field lines.Plain Language SummaryAs society becomes more dependent on technologies susceptible to adverse space weather, it is becoming increasingly critical to have numerical models capable of running in real time to nowcast/forecast the conditions in the near‐Earth space environment. One such model is available at the Community Coordinated Modeling Center and has been running for several years, allowing for an assessment of the quality of the result. Comparisons are made against globally compiled index of near‐Earth space storm activity, including numerous statistical quantities and tests. The skill of the model is remarkable, especially when a few hours after each of the cold restarts of the model are removed from the comparison. It is also shown that a global model alone is not that good at reproducing this storm index; a regional model for the inner part of geospace is necessary for good data‐model agreement.Key PointsThe SWMF model has been running in experimental real‐time mode at CCMC for several years, and all saved output is availableThe comparison against real‐time Dst is quite good, especially when a few hours after cold restarts are removed from the comparisonIt is necessary to include an inner magnetospheric drift physics model to reproduce Dst; a real‐time run without one does much worsePeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/146631/1/swe20766.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/146631/2/swe20766_am.pd