Quantum mechanics

Magnetic field

Computational physics

Computer security

RADIUS

Astrophysics

Energy (signal processing)

Magnetic reconnection

Plasma

Electric field

Computer science

Classical mechanics

Plasmoid

Physics

Particle acceleration

Acceleration

Lorentz transformation

Lorentz factor

The Astrophysical Journal

Magnetic reconnection is invoked as one of the primary mechanisms to produce energetic particles. We employ large-scale 3D particle-in-cell simulations of reconnection in magnetically dominated ( σ = 10) pair plasmas to study the energization physics of high-energy 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 cross-sectional 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 large-scale fields in the upstream region. These “free” particles preferentially move in z along Speiser-like orbits sampling both sides of the layer and are accelerated linearly in time—their Lorentz factor scales as γ ∝ t , in contrast to <?CDATA $\gamma \propto \sqrt{t}$?> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>γ</mml:mi> <mml:mo>∝</mml:mo> <mml:msqrt> <mml:mrow> <mml:mi>t</mml:mi> </mml:mrow> </mml:msqrt> </mml:math> 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, <?CDATA ${{dN}}_{\mathrm{free}}/d\gamma \propto {\gamma }^{-1.5}$?> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi mathvariant="italic">dN</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>free</mml:mi> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi>d</mml:mi> <mml:mi>γ</mml:mi> <mml:mo>∝</mml:mo> <mml:msup> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.5</mml:mn> </mml:mrow> </mml:msup> </mml:math> , contains ∼20% of the dissipated magnetic energy independently of domain size, and extends up to a cutoff energy scaling linearly with box size. Our results demonstrate that relativistic reconnection in GRB and AGN jets may be a promising mechanism for generating ultra-high-energy cosmic rays.

Fast Particle Acceleration in Three-dimensional Relativistic Reconnection