Transition from slow to fast tearing instability in the kinetic regime: theory and simulations

Current sheets are ubiquitous in space and astrophysical plasmas, and may become unstable, under appropriate conditions, to the so-called tearing instability, which in turn triggers magnetic reconnection and massive magnetic to kinetic energy conversion.

Over the past decade, significant progress has been made on the initiation of fast reconnection via plasmoid instability in resistive plasmas. However, given the extremely high Lundquist numbers in most natural plasmas, the role of kinetic effects in breaking the frozen-in condition and triggering tearing instability has to be considered.

In this work, we observe by means of fully kinetic Particle In Cell simulations the onset of a fast tearing instability (e-folding time τ∼τ_A, with τ_A the Alfven time) in current sheets (CS) with thickness a>d_i, with d_i the ion skin depth.

We adopt the model for the transition from slow to fast tearing instability described in Pucci et al 2017 in the kinetic regime and in a double CS framework, where the electron inertial term replaces the resistive term as the dominant non ideal term in the generalised Ohm’s law in the vicinity of the neutral line. 

We show that the simulations verify the model predictions on the critical current sheet aspect ratio (thickness to length) below which the tearing instability growth rate is expected to approach the Alfven time (“fast tearing”).