Particle Acceleration in Turbulence and Weakly Stochastic Reconnection

TL;DR

This paper investigates how particles gain energy in different magnetic reconnection environments, showing turbulence enhances acceleration via a first-order Fermi process, while pure turbulence leads to a second-order process.

Contribution

It provides a detailed analysis of particle acceleration mechanisms in 3D MHD simulations, highlighting the impact of turbulence on acceleration rates and processes.

Findings

Turbulence significantly increases acceleration rates in reconnection zones.

Reconnection with turbulence results in a first-order Fermi acceleration.

Pure turbulence leads to a second-order Fermi acceleration mechanism.

Abstract

Fast particles are accelerated in astrophysical environments by a variety of processes. Acceleration in reconnection sites has attracted the attention of researchers recently. In this letter we analyze the energy distribution evolution of test particles injected in three dimensional (3D) magnetohydrodynamic (MHD) simulations of different magnetic reconnection configurations. When considering a single Sweet-Parker topology, the particles accelerate predominantly through a first-order Fermi process, as predicted in previous work (de Gouveia Dal Pino & Lazarian, 2005) and demonstrated numerically in Kowal, de Gouveia Dal Pino & Lazarian (2011). When turbulence is included within the current sheet, the acceleration rate, which depends on the reconnection rate, is highly enhanced. This is because reconnection in the presence of turbulence becomes fast and independent of resistivity (Lazarian & Vishniac, 1999; Kowal et al., 2009) and allows the formation of a thick volume filled with multiple simultaneously reconnecting magnetic fluxes. Charged particles trapped within this volume suffer several head-on scatterings with the contracting magnetic fluctuations, which significantly increase the acceleration rate and results in a first-order Fermi process. For comparison, we also tested acceleration in MHD turbulence, where particles suffer collisions with approaching and receding magnetic irregularities, resulting in a reduced acceleration rate. We argue that the dominant acceleration mechanism approaches a second order Fermi process in this case.