System Size Dependence of Collisionless Reconnection Rate

TL;DR

This paper shows that the collisionless magnetic reconnection rate depends on system size when the initial conditions are scaled self-consistently, challenging the idea of a universal reconnection rate.

Contribution

It provides a comprehensive scaling study demonstrating the size dependence of reconnection rate across different magnetic configurations.

Findings

Reconnection rate decreases with increasing system size under self-consistent scaling.

Universal fast reconnection rate is not maintained when initial current sheet thickness scales with system size.

Size dependence is a fundamental property of collisionless reconnection, not limited to specific geometries.

Abstract

It is a widely accepted paradigm that collisionless magnetic reconnection proceeds at a universal fast rate of $\sim0.1$ when normalized to a properly defined reconnecting magnetic field and Alfv\'en speed, effectively independent of the macroscopic system size. This conclusion, derived primarily from kinetic simulations of classical Harris current sheets with kinetic-scale thickness, stands in contrast to results from forced reconnection and island coalescence, where the rate significantly depends on the system size. Here, we reconcile this disparity by performing a rigorous scaling study using both particle-in-cell and Hall magnetohydrodynamic simulations. We demonstrate that when the global magnetic configuration is self-consistently preserved by scaling the initial current sheet thickness proportionally with the system size, the ``universal'' fast rate disappears. Instead, the reconnection rate decreases as the system size increases. These results indicate that dependence on macroscopic scales is not peculiar to specific geometries but is a fundamental property of collisionless reconnection, effectively unifying the Harris sheet with other configurations exhibiting size-dependence.