Determining the Dominant Acceleration Mechanism during Relativistic Magnetic Reconnection in Large-scale Systems

Abstract

While a growing body of research indicates that relativistic magnetic reconnection is a prodigious source of particle acceleration in high-energy astrophysical systems, the dominant acceleration mechanism remains controversial. Using a combination of fully kinetic simulations and theoretical analysis, we demonstrate that Fermi-type acceleration within the large-scale motional electric fields dominates over direct acceleration from non-ideal electric fields within small-scale diffusion regions. This result has profound implications for modeling particle acceleration in large-scale astrophysical problems, since it opens up the possiblity of modeling the energetic spectra without resolving microscopic diffusion regions.

Second-order Fermi Reacceleration Mechanisms and Large-Scale Synchrotron Radio Emission in Intracluster Bridges

“…More recently, Brunetti and Lazarian [37] proposed a mechanism that operates in large-scale super-Alfvénic solenoidal turbulence in the ICM, where particles are reaccelerated stochastically diffusing across regions of magnetic reconnection and dynamo [see 38,39, for application to gamma-ray bursts and Pulsar wind nebulae]. On much smaller scales, situations involving first order and second order Fermi-like acceleration are also observed in simulations of reconnection regions [e.g., [40][41][42][43]. In the case of prevalence of solenoidal component and strongly super-Alfvénic turbulence,…”

Formation of Power-law Electron Energy Spectra in Three-dimensional Low-β Magnetic Reconnection

“…Recent kinetic simulations of magnetic reconnection in the relativistic regime have shown the formation of power-law energy spectra (e.g. Guo et al 2014Guo et al , 2015Guo et al , 2019Sironi & Spitkovsky 2014;Werner et al 2016). However, obtaining power-law distributions in the nonrelativistic regime relevant to solar flares is considerably more difficult.…”

Studying particle acceleration from driven magnetic reconnection at the termination shock of a relativistic striped wind using particle-in-cell simulations

“…Sironi and Spitkovsky [26] have suggested that the power-law forms as the particles interact with the X-points (diffusion regions with weak magnetic field |E| > |B|) through direct acceleration. In contrast, analyses by Guo et al [25,27,29] show that the power-law distributions are produced by Fermi-like processes and continuous injection from the reconnection inflow. In the case of the shockdriven reconnection at the termination shocks of highly relativistic striped pulsar winds, Sironi and Spitkovsky [20] have proposed that high-energy particles are mainly accelerated at the electric fields at the X-points.…”

“…For such local frames, we can decompose the electric fields based on Lorentz transformation, with detailed derivations given in the Appendix C 1 and C 2. The ways we decompose the electric fields are a generalization for the non-relativistic case used extensively in previous studies [25,27,29,37]. This generalization is critical in this work because the motion of the bulk flow in our simulations can be highly relativistic.…”

“…Numerical simulations including magnetohydrodynamics (MHD) [5,[17][18][19], particle-in-cell (PIC) [20] and test-particle simulations [21] have been used to model the termination shock of PWNe. However, how magnetic energy is converted, and the role of the termination shock [22][23][24] and relativistic magnetic reconnection [25][26][27][28][29] in accelerating par-ticles is still unclear [2]. Magnetic reconnection driven by the termination shock may dissipate the magnetic energy and accelerate particles [20,30].…”