Particle Acceleration and Plasma Dynamics During Magnetic Reconnection in the Magnetically Dominated Regime

Abstract

Magnetic reconnection is thought to be the driver for many explosive phenomena in the universe. The energy release and particle acceleration during reconnection have been proposed as a mechanism for producing high-energy emissions and cosmic rays. We carry out two-and three-dimensional kinetic simulations to investigate relativistic magnetic reconnection and the associated particle acceleration. The simulations focus on electron-positron plasmas starting with a magnetically dominated, force-free current sheet (σ ≡ B 2 /(4πn e m e c 2 ) 1). For this limit, we demonstrate that relativistic reconnection is highly efficient at accelerating particles through a first-order Fermi process accomplished by the curvature drift of particles along the electric field induced by the relativistic flows. This mechanism gives rise to the formation of hard power-law spectra f ∝ (γ − 1) −p and approaches p = 1 for sufficiently large σ and system size. Eventually most of the available magnetic free energy is converted into nonthermal particle kinetic energy. An analytic model is presented to explain the key results and predict a general condition for the formation of power-law distributions. The development of reconnection in these regimes leads to relativistic inflow and outflow speeds and enhanced reconnection rates relative to non-relativistic regimes. In the three-dimensional simulation, the interplay between secondary kink and tearing instabilities leads to strong magnetic turbulence, but does not significantly change the energy conversion, reconnection rate, or particle acceleration. This study suggests that relativistic reconnection sites are strong sources of nonthermal particles, which may have important implications to a variety of high-energy astrophysical problems.

Polarization Signatures of Relativistic Magnetohydrodynamic Shocks in the Blazar Emission Region. I. Force-Free Helical Magnetic Fields

“…This can provide extra nonthermal particles, thus a slow shock in a partially turbulent, partially ordered (such as a helical geometry) magnetic field structure may generate the necessary nonthermal electrons. Several models investigating the reconnection in a highly magnetized environment have been proposed to explain very fast blazar events (Giannios et al 2009;Deng et al 2015;Guo et al 2015). Meanwhile, recent simultaneous fittings of blazar SEDs, light curves and polarization signatures also favor a reconnection process (Chandra et al 2015;Zhang et al 2015).…”

Ultrahigh-energy cosmic ray production by turbulence in gamma-ray burst jets and cosmogenic neutrinos

“…The induced turbulence can scatter charged particles, which causes the second order Fermi acceleration [36,[61][62][63]. In addition, the turbulence may enhance the rate of magnetic reconnection [64], which can also accelerate particles stochastically [65][66][67][68]. Although there are many candidates, we do not specify the particular (magneto-)hydrodynamical instability that is responsible for particle acceleration.…”

Particle acceleration during magnetic reconnection in a low-beta pair plasma

“…An important problem that remains unsolved is the acceleration of nonthermal charged particles in the reconnection region. While observations have shown strong evidence of particle acceleration associated with magnetic reconnection [5][6][7], the primary acceleration mechanism is still under debate [8][9][10][11][12][13][14][15][16][17][18][19][20][21]. It is worthwhile to point out that the acceleration mechanism may depend critically on how reconnection actually proceeds in large 3D system, a subject which is currently an active area of research.…”

“…However, effects like MHD turbulence and 3D physics that could be important to the physics of magnetic reconnection have not been fully understood to reach a consensus [19,[26][27][28][29][30][31][32].…”