Relativistic Nonthermal Particle Acceleration in Two-Dimensional Collisionless Magnetic Reconnection

TL;DR

This paper presents an analytical model explaining how relativistic magnetic reconnection accelerates particles to nonthermal energies, linking the particle spectrum's features to plasma magnetization, system size, and plasmoid dynamics.

Contribution

The paper introduces a simple self-similar analytical model that explains the dependencies of particle energy spectra on plasma parameters in relativistic reconnection.

Findings

Power-law index depends on the balance between electric acceleration and magnetization.

High-energy cutoff is governed by particle trapping in large plasmoids.

Model aligns with numerical observations of particle spectra in reconnection.

Abstract

Magnetic reconnection, especially in the relativistic regime, provides an efficient mechanism for accelerating relativistic particles and thus offers an attractive physical explanation for nonthermal high-energy emission from various astrophysical sources. I present a simple analytical model that elucidates key physical processes responsible for reconnection-driven relativistic nonthermal particle acceleration (NTPA) in the large-system, plasmoid-dominated regime in two dimensions. The model aims to explain the numerically-observed dependencies of the power-law index $p$ and high-energy cutoff $\gamma_c$ of the resulting nonthermal particle energy spectrum $f(\gamma)$ on the ambient plasma magnetization $\sigma$, and (for $\gamma_c$) on the system size $L$. In this self-similar model, energetic particles are continuously accelerated by the out-of-plane reconnection electric field $E_{\rm rec}$ until they become magnetized by the reconnected magnetic field and eventually trapped in plasmoids large enough to confine them. The model also includes diffusive Fermi acceleration by particle bouncing off rapidly moving plasmoids. I argue that the balance between electric acceleration and magnetization controls the power-law index, while trapping in plasmoids governs the cutoff, thus tying the particle energy spectrum to the plasmoid distribution.