The island coalescence problem: scaling of reconnection in extended fluid models including higher-order moments

TL;DR

This paper investigates how different fluid models, including a ten-moment model, can be used to understand the scaling of magnetic reconnection in space plasmas, highlighting the limitations of Hall MHD and the potential of higher-order moment models.

Contribution

The study compares resistive MHD, Hall MHD, and a ten-moment fluid model to assess their effectiveness in capturing reconnection physics in island coalescence, especially at large scales.

Findings

Hall MHD fails to fully capture reconnection physics.

The ten-moment model shows promise in reproducing ion kinetic effects.

Reconnection rate scaling varies with the model used.

Abstract

As modeling of collisionless magnetic reconnection in most space plasmas with realistic parameters is beyond the capability of today's simulations, due to the separation between global and kinetic length scales, it is important to establish scaling relations in model problems so as to extrapolate to realistic scales. Recently, large scale particle-in-cell (PIC) simulations of island coalescence have shown that the time averaged reconnection rate decreases with system size, while fluid systems at such large scales in the Hall regime have not been studied. Here we perform the complementary resistive MHD, Hall MHD and two fluid simulations using a ten-moment model with the same geometry. In contrast to the standard Harris sheet reconnection problem, Hall MHD is insufficient to capture the physics of the reconnection region. Additionally, motivated by the results of a recent set of hybrid simulations which show the importance of ion kinetics in this geometry, we evaluate the efficacy of the ten-moment model in reproducing such results.