Stochastic acceleration by multi-island contraction during turbulent magnetic reconnection

TL;DR

This paper presents a stochastic model of particle acceleration during turbulent magnetic reconnection, highlighting the roles of both systematic and random compression effects in energizing particles.

Contribution

It introduces a combined first and second-order Fermi acceleration model that accounts for statistical fluctuations in compression rates during multi-island reconnection.

Findings

Second-order stochastic acceleration can dominate systematic acceleration when compression rate variance is large.

The model explains features observed in numerical simulations of particle energization.

Both Eulerian and Lagrangian properties influence acceleration efficiency.

Abstract

The acceleration of charged particles in magnetized plasmas is considered during turbulent multi-island magnetic reconnection. The particle acceleration model is constructed for an ensemble of islands which produce adiabatic compression of the particles. The model takes into account the statistical fluctuations in the compression rate experienced by the particles during their transport in the acceleration region. The evolution of the particle distribution function is described as a simultaneous first and second-order Fermi acceleration process. While the efficiency of the first-order process is controlled by the average rate of compression, the second order process involves the variance in the compression rate. Moreover, the acceleration efficiency associated with the second-order process involves both the Eulerian properties of the compression field and the Lagrangian properties of the particles. The stochastic contribution to the acceleration is non-resonant and can dominate the systematic part in the case of a large variance in the compression rate. The model addresses the role of the second-order process, how the latter can be related to the large-scale turbulent transport of particles and explains some features of the numerical simulations of particle acceleration by multi-island contraction during magnetic reconnection.