Development of Tearing Instability in a Current Sheet Forming by Sheared Incompressible Flow

TL;DR

This paper extends tearing mode theory to analyze the stability of current sheets forming under sheared incompressible flows, revealing that standard scalings can be adapted with minor modifications for such dynamic systems.

Contribution

It develops a modified tearing mode analysis framework for current sheets forming under flow, incorporating time dependence and flow effects into standard theory.

Findings

Flow influences perturbation growth implicitly through time-dependent scalings.

Standard tearing theory can be adapted for forming sheets with minor modifications.

Flow effects can be incorporated into nonlinear Rutherford stage analysis.

Abstract

Sweet-Parker current sheets in high Lundquist number plasmas are unstable to tearing, suggesting they will not form in physical systems. Understanding magnetic reconnection thus requires study of the stability of a current sheet as it forms. Formation can occur due to sheared, sub-Alfv\'enic incompressible flows which narrow the sheet. Standard tearing theory (Furth et al. 1963; Coppi et al. 1976; Rutherford 1973) is not immediately applicable to such forming sheets for two reasons: first, because the flow introduces terms not present in the standard calculation; second, because the changing equilibrium introduces time dependence to terms which are constant in the standard calculation, complicating the formulation of an eigenvalue problem. This paper adapts standard tearing mode analysis to confront these challenges. In an initial phase when any perturbations are primarily governed by ideal MHD, a coordinate transformation reveals that the flow compresses and stretches perturbations. A multiple scale formulation describes how linear tearing mode theory (Furth et al. 1963; Coppi et al. 1976) can be applied to an equilibrium changing under flow, showing that the flow affects the separable exponential growth only implicitly, by making the standard scalings time-dependent. In the nonlinear Rutherford stage, the coordinate transformation shows that standard theory can be adapted by adding to the stationary rates time dependence and an additional term due to the strengthening equilibrium magnetic field. Overall, this understanding supports the use of flow-free scalings with slight modifications to study tearing in a forming sheet.