The role of guide field in magnetic reconnection driven by island coalescence

TL;DR

This study investigates how a guide magnetic field influences the rate and dynamics of magnetic reconnection during island merging, highlighting the importance of ion kinetic physics and the limitations of Hall-MHD models.

Contribution

It reveals the specific conditions under which guide fields affect reconnection rates and demonstrates the necessity of kinetic physics for accurate modeling.

Findings

Reconnection rate increases with guide field within a limited range.

Hall-MHD models match kinetic rates only at strong guide fields.

Zero guide field merging is slower, possibly due to upstream instabilities.

Abstract

A number of studies have considered how the rate of magnetic reconnection scales in large and weakly collisional systems by the modelling of long reconnecting current sheets. However, this set-up neglects both the formation of the current sheet and the coupling between the diffusion region and a larger system that supplies the magnetic flux. Recent studies of magnetic island merging, which naturally include these features, have found that ion kinetic physics is crucial to describe the reconnection rate and global evolution of such systems. In this paper, the effect of a guide field on reconnection during island merging is considered. In contrast to the earlier current sheet studies, we identify a limited range of guide fields for which the reconnection rate, outflow velocity, and pile-up magnetic field increase in magnitude as the guide field increases. The Hall-MHD fluid model is found to reproduce kinetic reconnection rates only for a sufficiently strong guide field, for which ion inertia breaks the frozen-in condition and the outflow becomes Alfvenic in the kinetic system. The merging of large islands occurs on a longer timescale in the zero guide field limit, which may in part be due to a mirror-like instability that occurs upstream of the reconnection region.