The interplay of magnetically-dominated turbulence and magnetic reconnection in producing nonthermal particles

Luca Comisso; Lorenzo Sironi

doi:10.3847/1538-4357/ab4c33

paper

The interplay of magnetically-dominated turbulence and magnetic reconnection in producing nonthermal particles

arXiv:1909.01420 · doi:10.3847/1538-4357/ab4c33

Abstract

Magnetized turbulence and magnetic reconnection are often invoked to explain the nonthermal emission observed from a wide variety of astrophysical sources. By means of fully-kinetic 2D and 3D PIC simulations, we investigate the interplay between turbulence and reconnection in generating nonthermal particles in magnetically-dominated pair plasmas. A generic by-product of the turbulence evolution is the generation of a nonthermal particle spectrum with a power-law energy range. The power-law slope $p$ is harder for larger magnetizations and stronger turbulence fluctuations, and it can be as hard as $p < 2$ . The Larmor radius of particles at the high-energy cutoff is comparable to the size $l$ of the largest eddies. Plasmoid-mediated reconnection, which self-consistently occurs in the turbulent plasma, controls the physics of particle injection. Then, particles are further accelerated by stochastic scattering off turbulent fluctuations. The work done by parallel electric fields - naturally expected in reconnection layers - is responsible for most of the initial energy increase, and is proportional to the magnetization $σ$ of the system, while the subsequent energy gain, which dominates the overall energization of high-energy particles, is powered by the perpendicular electric fields of turbulent fluctuations. The two-stage acceleration process leaves an imprint in the particle pitch-angle distribution: low-energy particles are aligned with the field, while the highest energy particles move preferentially orthogonal to it. The energy diffusion coefficient of stochastic acceleration scales as $D_{γ} \sim 0.1 σ (c / l) γ^{2}$ , where $γ$ is the particle Lorentz factor. This results in fast acceleration timescales $t_{a cc} \sim (3/ σ) l / c$ . Our findings have important implications for understanding the generation of nonthermal particles in high-energy astrophysical sources.

arXiv commentAnimations are available at: https://doi.org/10.7916/d8-prt9-kn88

The interplay of magnetically-dominated turbulence and magnetic reconnection in producing nonthermal particles

Abstract

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