Dynamo-driven plasmoid formation from a current-sheet instability

TL;DR

This paper demonstrates how dynamo-driven flux amplification at current sheets can trigger plasmoid formation through a tearing instability, revealing a new mechanism for fast magnetic reconnection in plasmas.

Contribution

It introduces the first observation of dynamo-driven flux amplification leading to plasmoid formation in 3D resistive MHD simulations, linking flux amplification to reconnection dynamics.

Findings

Nonaxisymmetric fluctuations grow rapidly at the edge current sheet.

Dynamo flux amplification occurs at the reconnection site.

Flux amplification triggers plasmoid instability and current sheet breakup.

Abstract

Axisymmetric current-carrying plasmoids are formed in the presence of nonaxisymmetric fluctuations during nonlinear three-dimensional resistive MHD simulations in a global toroidal geometry. We utilize the helicity injection technique to form an initial poloidal flux in the presence of a toroidal guide field. As helicity is injected, two types of current sheets are formed from 1) the oppositely directed field lines in the injector region (primary reconnecting current sheet), and 2) the poloidal flux compression near the plasma edge (edge current sheet). We first find that nonaxisymmetic fluctuations arising from the current-sheet instability isolated near the plasma edge have tearing parity but can nevertheless grow fast (on the poloidal Alfven time scale). These modes saturate by breaking up the current sheet. Second, for the first time a dynamo poloidal flux amplification is observed at the reconnetion site (in the region of the oppositely directed magnetic field). This fluctuation-induced flux amplification increases the local Lundquist number, which then triggers a plasmoid instability and breaks the primary current sheet at the reconnection site. The plasmoids formation driven by large-scale flux amplification, i.e. a large-scale dynamo, observed here has strong implications for astrophysical reconnection as well as fast reconnection events in laboratory plasmas.