Explosive magnetic reconnection caused by an X-shaped current-vortex layer in a collisionless plasma

TL;DR

This paper investigates a new explosive magnetic reconnection mechanism in collisionless plasmas, driven by an X-shaped current-vortex layer, combining high-resolution simulations and a novel theoretical model.

Contribution

It introduces a new local reconnection model with an X-shaped layer, explaining explosive growth and energy release in collisionless plasma reconnection.

Findings

Explosive growth occurs locally around the reconnection point.

The X-shaped layer's length is shorter than the domain and linear mode wavelength.

Theoretical scaling matches simulation results.

Abstract

A mechanism for explosive magnetic reconnection is investigated by analyzing the nonlinear evolution of a collisionless tearing mode in a two-fluid model that includes the effects of electron inertia and temperature. These effects cooperatively enable a fast reconnection by forming an X-shaped current-vortex layer centered at the reconnection point. A high-resolution simulation of this model for an unprecedentedly small electron skin depth $d_e$ and ion-sound gyroradius $\rho_s$, satisfying $d_e=\rho_s$, shows an explosive tendency for nonlinear growth of the tearing mode, where it is newly found that the explosive widening of the X-shaped layer occurs locally around the reconnection point with the length of the X shape being shorter than the domain length and the wavelength of the linear tearing mode. The reason for the onset of this locally enhanced reconnection is explained theoretically by developing a novel nonlinear and nonequilibrium inner solution that models the local X-shaped layer, and then matching it to an outer solution that is approximated by a linear tearing eigenmode with a shorter wavelength than the domain length. This theoretical model proves that the local reconnection can release the magnetic energy more efficiently than the global one and the estimated scaling of the explosive growth rate agrees well with the simulation results.