Abstract Magnetic reconnection is invoked as one of the primary mechanisms to produce energetic particles. We employ largescale 3D particleincell simulations of reconnection in magnetically dominated ( σ = 10) pair plasmas to study the energization physics of highenergy particles. We identify an acceleration mechanism that only operates in 3D. For weak guide fields, 3D plasmoids/flux ropes extend along the z direction of the electric current for a length comparable to their crosssectional radius. Unlike in 2D simulations, where particles are buried in plasmoids, in 3D we find that a fraction of particles with γ ≳ 3 σ can escape from plasmoids by moving along z , and so they can experience the largescale fields in the upstream region. These “free” particles preferentially move in z along Speiserlike orbits sampling both sides of the layer and are accelerated linearly in time—their Lorentz factor scales as γ ∝ t , in contrast to γ ∝ t in 2D. The energy gain rate approaches ∼ eE rec c , where E rec ≃ 0.1 B 0 is the reconnection electric field and B 0 the upstream magnetic field. The spectrum of free particles is hard, dN free / d γ ∝ γmore »
Relativistic nonthermal particle acceleration in twodimensional collisionless magnetic reconnection
Magnetic reconnection, especially in the relativistic regime, provides an efficient mechanism for accelerating relativistic particles and thus offers an attractive physical explanation for nonthermal highenergy emission from various astrophysical sources. I present a simple analytical model that elucidates key physical processes responsible for reconnectiondriven relativistic nonthermal particle acceleration in the largesystem, plasmoiddominated regime in two dimensions. The model aims to explain the numerically observed dependencies of the powerlaw index $p$ and highenergy 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 selfsimilar model, energetic particles are continuously accelerated by the outofplane 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 powerlaw index, while trapping in plasmoids governs the cutoff, thus tying the particle energy spectrum to the plasmoid distribution.
 Award ID(s):
 1903335
 Publication Date:
 NSFPAR ID:
 10330364
 Journal Name:
 Journal of Plasma Physics
 Volume:
 88
 Issue:
 1
 ISSN:
 00223778
 Sponsoring Org:
 National Science Foundation
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