Fast magnetic reconnection: The "ideal" tearing instability in classic, Hall, and relativistic plasmas

TL;DR

This paper investigates the rapid magnetic reconnection process across classical, Hall, and relativistic plasmas, demonstrating that thin current sheets lead to fast, ideal tearing instabilities capable of explaining explosive astrophysical phenomena.

Contribution

It presents the first 2D compressible MHD simulations of secondary tearing instabilities in Hall-MHD and summarizes relativistic tearing instability results, highlighting the universality of fast reconnection.

Findings

Reconnection becomes 'ideal' in thin current sheets, independent of Lundquist number.

Hall-MHD simulations reveal onset of secondary tearing instabilities.

Relativistic tearing instability is extremely fast, matching astrophysical timescales.

Abstract

Magnetic reconnection is believed to be the driver of many explosive phenomena in Astrophysics, from solar to gamma-ray flares in magnetars and in the Crab nebula. However, reconnection rates from classic MHD models are far too slow to explain such observations. Recently, it was realized that when a current sheet gets sufficiently thin, the reconnection rate of the tearing instability becomes "ideal", in the sense that the current sheet destabilizes on the "macroscopic" Alfv\'enic timescales, regardless of the Lundquist number of the plasma. Here we present 2D compressible MHD simulations in the classical, Hall, and relativistic regimes. In particular, the onset of secondary tearing instabilities is investigated within Hall-MHD for the first time. In the frame of relativistic MHD, we summarize the main results from Del Zanna et al. [1]: the relativistic tearing instability is found to be extremely fast, with reconnection rates of the order of the inverse of the light crossing time, as required to explain the high-energy explosive phenomena.