Magnetic dissipation in a force-free plasma with a sheet-pinch configuration

Abstract

This paper describes the study of a force-free plasma with an initial sheet-pinch configuration ∇×B=−αB, where α is a scalar constant, using both linear theory and 2-1/2 dimensional particle-in-cell simulations with doubly periodic boundary conditions. Previous studies have shown that this configuration is unstable to the collisionless tearing instability. In this work, the linear growth rate in the long wavelength limit is found to have an upper bound at 1/τA, where τA is the Alfvén traversal time through the region with sheared magnetic fields. The simulations show that the initial force-free state evolves by seeking a more “relaxed,” lower energy state within the constraints of the simulation geometry. Under the periodic boundary conditions, the amount of magnetic energy available for dissipation is determined mostly by the geometry. The reconnection region shows a multiscale structure, separated by ion and electron inertial lengths. Particle and flow dynamics at the reconnection regions have been analyzed. The reconnection rate is shown to be high, with an ion inflow speed of ∼0.2 Alfvén speed, and the timescale for reconnection is several Ωi−1. Most of the dissipated magnetic energy goes into the thermal energy of the particles.