Particle Acceleration in Magnetic Reconnection with Ad hoc Pitch-angle Scattering

TL;DR

This paper introduces a method using pitch-angle scattering in 2D simulations to mimic 3D particle acceleration effects in magnetic reconnection, enhancing energetic particle production.

Contribution

It demonstrates that ad hoc pitch-angle scattering in 2D simulations can replicate 3D particle escape and acceleration phenomena, providing a cost-effective alternative.

Findings

Scattered particles can escape 2D magnetic islands and undergo enhanced Fermi acceleration.

Increased scattering frequency leads to more energetic, nonthermal particle spectra.

The method bridges the gap between 2D and 3D simulation results in magnetic reconnection studies.

Abstract

Particle acceleration during magnetic reconnection is a long-standing topic in space, solar and astrophysical plasmas. Recent 3D particle-in-cell simulations of magnetic reconnection show that particles can leave flux ropes due to 3D field-line chaos, allowing particles to access additional acceleration sites, gain more energy through Fermi acceleration, and develop a power-law energy distribution. This 3D effect does not exist in traditional 2D simulations, where particles are artificially confined to magnetic islands due to their restricted motions across field lines. Full 3D simulations, however, are prohibitively expensive for most studies. Here, we attempt to reproduce 3D results in 2D simulations by introducing ad hoc pitch-angle scattering to a small fraction of the particles. We show that scattered particles are able to transport out of 2D islands and achieve more efficient Fermi acceleration, leading to a significant increase of energetic particle flux. We also study how the scattering frequency influences the nonthermal particle spectra. This study helps achieve a complete picture of particle acceleration in magnetic reconnection.