Numerical Methods for Plasma Astrophysics:
From Particle Kinetics to MHD

CSIC Building (#406), Seminar Room 4122.
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Kinetic Modeling of Magnetic Field Dynamics in Space and Astrophysical Systems

Dr. Jim Drake

Department of Physics and Institute of Physical Sci & Technology at University of Maryland

Abstract:   The large scale dynamics of magnetized plasma systems are typically modeled with the MHD equations. However, the MHD description typically breaks down at spatial scales where dissipation is required to either break magnetic field lines, allowing reconnection to occur, or to locally dissipate energy in the form of magnetic fields or macroscopic flows. In the case of magnetic reconnection, the Hall MHD model has been found to accurately reproduce the rates of reconnection determined by kinetic modeling, a consequence of the role of dispersive waves in reconnection. However, critical issues in space and astrophysics remain that require a kinetic description and at the same time have significant consequences for the description of the large-scale dynamics of plasma systems. I will focus on two generic topics, electron heating and kinetic scale turbulence, to illustrate the conceptual challenges and to highlight the importance of kinetic modeling. Observational data from the Sun and the Earth's magnetosphere and auroral ionosphere provide important data that has broad implications. Nearly half of the magnetic energy released in solar flares is channeled into energetic electrons and recent observations in the magnetosphere confirm that reconnection can directly drive electrons to near relativistic energies. The mechanism is unknown. In boundary layers of the magnetosphere, where large-scale parallel electric fields are expected from modeling, parallel electric fields take the form of intense, spatially-localized, bipolar structures (electron holes) and double-layers. These are manifestly kinetic nonlinear structures where electrons and ions can directly exchange energy with large scale fields. How to provide kinetic input to the large scale modeling of plasma systems has become a central issue. Because of the enormous range of time scales, conventional AMR techniques are not likely to be sufficient. New ideas such as 'projective integration' that retain the full kinetic dynamics while offering the possibility of time advancing large scale plasma systems seem worthy of exploration.