Alfvénic collisionless magnetic reconnection and the Hall term

TLDR

The authors simulate the GEM Harris current sheet in 2.5D with full particle, hybrid, and Hall MHD models, showing that whistler‑wave dynamics at ion‑unmagnetized scales make the reconnection process insensitive to the specific mechanism that breaks the frozen‑in condition. All simulations produce the same gross reconnection rate when the Hall term is included, indicating that the rate is independent of the frozen‑in‑break mechanism, that Hall MHD alone suffices for correct rates, and that a downstream density depletion linked to Hall currents is observed.

Abstract

The Geospace Environment Modeling (GEM) Challenge Harris current sheet problem is simulated in 2 1/2 dimensions using full particle, hybrid, and Hall MHD simulations. The same gross reconnection rate is found in all of the simulations independent of the type of code used, as long as the Hall term is included. In addition, the reconnection rate is independent of the mechanism which breaks the frozen‐in flux condition, whether it is electron inertia or grid scale diffusion. The insensitivity to the mechanism which breaks the frozen‐in condition is a consequence of whistler waves, which control the plasma dynamics at the small scales where the ions become unmagnetized. The dispersive character of whistlers, in which the phase velocity increases with decreasing scale size, allows the flux of electrons flowing away from the dissipation region to remain finite even as the strength of the dissipation approaches zero. As a consequence, the throttling of the reconnection process as a result of the small scale size of the dissipation region, which occurs in the magnetohydrodynamic model, iio longer takes place. The important consequence is that the minimum physical model necessary to produce physically correct reconnection rates is a Hall MHD description which includes the Hall term in Ohm's law. A density depletion layer, which lies just downstream from the magnetic separatrix, is identified and linked to the strong in‐plane Hall currents which characterize kinetic models of magnetic reconnection.