"Ideally" unstable current sheets and the triggering of fast magnetic reconnection

Abstract

Magnetic reconnection is thought to be the dynamical mechanism underlying many explosive phenomena observed both in space and in the laboratory, though the question of how fast magnetic reconnection is triggered in such high Lundquist ($S$) number plasmas has remained elusive. It has been well established that reconnection can develop over timescales faster than those predicted traditionally once kinetic scales are reached. It has also been shown that, within the framework of resistive Magnetohydrodynamics (MHD), fast reconnection is achieved for thin enough sheets via the onset of the so-called plasmoid instability. The latter was discovered in studies specifically devoted to the Sweet-Parker current sheet, either as an initial condition or an apparent transient state developing in nonlinear studies. On the other hand, a fast tearing instability can grow on an ideal, i.e., $S$-independent, timescale (dubbed "ideal" tearing) within current sheets whose aspect ratio scales with the macroscopic Lundquist number as $L/a\sim S^{1/3}$ -- much smaller than the Sweet-Parker one -- suggesting a new way to approach to the initiation of fast reconnection in collapsing current configurations. Here we present an overview of what we have called "ideal" tearing in resistive MHD, and discuss how the same reasoning can be extended to other plasma models commonly used that include electron inertia and kinetic effects. We then discuss a scenario for the onset of "ideal" fast reconnection via collapsing current sheets and describe a quantitative model for the interpretation of the nonlinear evolution of "ideally" unstable sheets in two dimensions.

Accepted for publication in JPP. Date of first submission: December 12, 2015

References in corpus (6)

• Instability of current sheets and formation of plasmoid chains
• Self-Feeding Turbulent Magnetic Reconnection on Macroscopic Scales
• A catastrophe model for fast magnetic reconnection onset
• Onset of Fast Reconnection in Hall Magnetohydrodynamics Mediated by the Plasmoid Instability
• The tearing mode instability of thin current sheets: the transition to fast reconnection in the presence of viscosity
• Extended theory of the Taylor problem in the plasmoid-unstable regime

Cited by in corpus (8)

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• Impact of 3D Structure on Magnetic Reconnection
• Viscous effects on plasmoid formation from nonlinear resistive tearing growth in a Harris sheet
• Activation of MHD reconnection on ideal timescales
• Optimal Energy Growth in Current Sheets
• Stability of the magnetotail current sheet with normal magnetic field and field-aligned plasma flows