Stochastic Particle Acceleration in Multiple Magnetic Islands during Reconnection

TL;DR

This paper proposes a new particle acceleration mechanism during magnetic reconnection involving multiple magnetic islands, which enhances acceleration efficiency from second-order to first-order in the Alfvén velocity over the speed of light.

Contribution

It introduces a novel acceleration process involving multiple magnetic islands during reconnection, improving efficiency compared to the traditional Fermi model.

Findings

Acceleration efficiency is first-order in $V_A/c$ during reconnection.

Particles tend to be distributed outside magnetic islands, interacting with outflow jets.

The mechanism offers a more efficient acceleration process than the original Fermi model.

Abstract

A nonthermal particle acceleration mechanism involving the interaction of a charged particle with multiple magnetic islands is proposed. The original Fermi acceleration model, which assumes randomly distributed magnetic clouds moving at random velocity $V_c$ in the interstellar medium, is known to be of second-order acceleration of $O(V_c/c)^2$ owing to the combination of head-on and head-tail collisions. In this letter, we reconsider the original Fermi model by introducing multiple magnetic islands during reconnect ion instead of magnetic clouds. We discuss that the energetic particles have a tendency to be distributed outside the magnetic islands, and they mainly interact with reconnection outflow jets. As a result, the acceleration efficiency becomes first-order of $O(V_A/c)$, where $V_A$ and $c$ are the Alfv\'en velocity and the speed of light, respectively.