Magnetic reconnection: Self-similar current sheet collapse triggered by "ideal" tearing

TL;DR

This paper uses MHD simulations to demonstrate that fast magnetic reconnection occurs via a self-similar, recursive tearing process triggered by the ideal tearing mode at high Lundquist numbers, leading to rapid current sheet disruption.

Contribution

It provides the first detailed simulation-based evidence of a hierarchical, self-similar tearing process driven by the ideal tearing mode in high Lundquist number plasmas.

Findings

Fast reconnection occurs before Sweet-Parker sheets form.

Hierarchy of tearing events accelerates at smaller scales.

Nonlinear recursive evolution explains current sheet disruption timescale.

Abstract

We study, by means of MHD simulations, the onset and evolution of fast reconnection via the "ideal" tearing mode within a collapsing current sheet at high Lundquist numbers ($S\gg10^4$). We first confirm that as the collapse proceeds, fast reconnection is triggered well before a Sweet-Parker type configuration can form: during the linear stage plasmoids rapidly grow in a few Alfv\'en times when the predicted "ideal" tearing threshold $S^{-1/3}$ is approached from above; after the linear phase of the initial instability, X-points collapse and reform nonlinearly. We show that these give rise to a hierarchy of tearing events repeating faster and faster on current sheets at ever smaller scales, corresponding to the triggering of "ideal" tearing at the renormalized Lundquist number. In resistive MHD this process should end with the formation of sub-critical ($S \leq10^4$) Sweet Parker sheets at microscopic scales. We present a simple model describing the nonlinear recursive evolution which explains the timescale of the disruption of the initial sheet.