GEM: The Role of Magnetized Electrons in Geospace Magnetic Reconnection

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

This project is funded under the Geospace Environment Modeling (GEM) program,a broad-based, community-initiated research program on the physics of the Earth's magnetosphere and the coupling of the magnetosphere to the atmosphere and to the solar wind. The purpose of the GEM program is to support basic research into the dynamical and structural properties of geospace, leading to the construction of a global Geospace General Circulation Model (GGCM) with predictive capability. Reconnection is the process by which stress in the magnetic field of magnetized plasma is reduced by rearrangement of the magnetic field. It is an important process in naturally occurring plasmas found throughout a wide range of astrophysical environments. At the Earth it is the primary physical mechanism for transport of mass, momentum and energy from the solar wind to the magnetosphere. It is applicable to both the dayside magnetopause and the magnetotail. Studying reconnection is difficult because it involves a wide range of spatial and temporal scale sizes from macroscopic (system wide) to microscopic (the electron inertial length). The only way to include all of the physics of reconnection is to use particle in cell (PIC) simulations and they require vast amounts of computer resources. As a result such simulations have been limited to very simple configurations and have not been used to study phenomena exhaustively. One simplifying approximation that is adopted is to use ion to electron mass ratios less than 1836 (256 is most frequently used now). In essence this means that the electrons are too massive. Recent results have suggested that lighter electrons are more easily magnetized by a guide (along the reconnection line) magnetic field, such that for this case the first adiabatic invariant is conserved. This, in turn, causes a phase-transition to a new regime where large scale electron current layers form in the diffusion layer and these extend to the system size. Even a small (12%) guide field will do this. The purpose of this project is to investigate this new regime. The large scale electron diffusion region is caused by electron pressure anisotropy. This can be accounted for with anisotropic equations of state. Therefore a two fluid simulation code that is much more efficient than a PIC code will be used to study the dynamics of large scale diffusion layers. However, the results from the multi-fluid code will be validated with full PIC simulations. The project will support the participation of a graduate student.