Geospace Environmental Modeling (GEM) Magnetic Reconnection Challenge

The Geospace Environmental Modeling (GEM) Reconnection Challenge project is presented and the important results, which are presented in a series of co mpanion papers, are summarized. Magnetic reconnection is studied in a simpl e Harris sheet configuration with a specified set of initial conditions, in cluding a finite amplitude, magnetic island perturbation to trigger the dyn amics. The evolution of the system is explored with a broad variety of code s, ranging from fully electromagnetic particle in cell (PIC) codes to conve ntional resistive magnetohydrodynamic (MHD) codes, and the results are comp ared. The goal is to identify the essential physics which is required to mo del collisionless magnetic reconnection. All models that include the Hall e ffect in the generalized Ohm's law produce essentially indistinguishable ra tes of reconnection, corresponding to nearly Alfvenic inflow velocities. Th us the rate of reconnection is insensitive to the specific mechanism which breaks the frozen-in condition, whether resistivity, electron inertia, or e lectron thermal motion. The reconnection rate in the conventional resistive MHD model, in contrast, is dramatically smaller unless a large localized o r current dependent resistivity is used. The Hall term brings the dynamics of whistler waves into the system. The quadratic dispersion property of whi stlers (higher phase speed at smaller spatial scales) is the key to underst anding these results. The implications of these results fbr trying to model the global dynamics of the magnetosphere are discussed.