Activation of MHD reconnection on ideal timescales

TL;DR

This paper demonstrates that magnetic reconnection can occur on ideal timescales in plasmas when the current sheet aspect ratio reaches a critical value, explaining rapid energy release observed in space and astrophysical phenomena.

Contribution

The study confirms that the critical aspect ratio of S^{1/3} triggers ideal timescale reconnection, extending understanding of tearing instability in visco-resistive MHD simulations.

Findings

Reconnection proceeds on Alfvénic timescales at critical aspect ratio.

Scaling applies to secondary reconnection events during nonlinear phase.

Robustness confirmed across inviscid and viscous regimes.

Abstract

Magnetic reconnection in laboratory, space and astrophysical plasmas is often invoked to explain explosive energy release and particle acceleration. However, the timescales involved in classical models within the macroscopic MHD regime are far too slow to match the observations. Here we revisit the tearing instability by performing visco-resistive two-dimensional numerical simulations of the evolution of thin current sheets, for a variety of initial configurations and of values of the Lunquist number $S$, up to $10^7$. Results confirm that when the critical aspect ratio of $S^{1/3}$ is reached in the reconnecting current sheets, the instability proceeds on ideal (Alfv\'enic) macroscopic timescales, as required to explain observations. Moreover, the same scaling is seen to apply also to the local, secondary reconnection events triggered during the nonlinear phase of the tearing instability, thus accelerating the cascading process to increasingly smaller spatial and temporal scales. The process appears to be robust, as the predicted scaling is measured both in inviscid simulations and when using a Prandtl number $P=1$ in the viscous regime.