Geospace Environmental Modeling (GEM) Magnetic Reconnection Challenge

TLDR

The GEM Reconnection Challenge aims to identify the essential physics needed to model collisionless magnetic reconnection. The study examines magnetic reconnection in a Harris sheet with an initial magnetic island perturbation, using a range of codes from PIC to resistive MHD to compare their evolution. Models that include the Hall term produce nearly Alfvénic, indistinguishable reconnection rates, showing the rate is insensitive to the specific frozen‑in breaking mechanism, whereas resistive MHD alone yields much smaller rates unless enhanced resistivity is used; the Hall term’s whistler dynamics and quadratic dispersion explain these results and have implications for global magnetospheric modeling.

Abstract

The Geospace Environmental Modeling (GEM) Reconnection Challenge project is presented and the important results, which are presented in a series of companion papers, are summarized. Magnetic reconnection is studied in a simple Harris sheet configuration with a specified set of initial conditions, including a finite amplitude, magnetic island perturbation to trigger the dynamics. The evolution of the system is explored with a broad variety of codes, ranging from fully electromagnetic particle in cell (PIC) codes to conventional resistive magnetohydrodynamic (MHD) codes, and the results are compared. The goal is to identify the essential physics which is required to model collisionless magnetic reconnection. All models that include the Hall effect in the generalized Ohm's law produce essentially indistinguishable rates of reconnection, corresponding to nearly Alfvénic inflow velocities. Thus the rate of reconnection is insensitive to the specific mechanism which breaks the frozen‐in condition, whether resistivity, electron inertia, or electron thermal motion. The reconnection rate in the conventional resistive MHD model, in contrast, is dramatically smaller unless a large localized or current dependent resistivity is used. The Hall term brings the dynamics of whistler waves into the system. The quadratic dispersion property of whistlers (higher phase speed at smaller spatial scales) is the key to understanding these results. The implications of these results for trying to model the global dynamics of the magnetosphere are discussed.