Concurrent Particle Acceleration and Pitch-angle Anisotropy Driven by Magnetic Reconnection: Ion-electron Plasmas

Abstract

Abstract Particle acceleration and pitch-angle anisotropy resulting from magnetic reconnection are investigated in highly magnetized ion-electron plasmas. By means of fully kinetic particle-in-cell simulations, we demonstrate that magnetic reconnection generates anisotropic particle distributions f s ∣ cos α ∣ , ε , characterized by broken power laws in the particle energy spectrum f s (ε) ∝ ε −p and pitch angle 〈 sin 2 α 〉 ∝ ε m . The characteristics of these distributions are determined by the relative strengths of the magnetic field’s guide and reconnecting components (B g /B 0) and the plasma magnetization (σ 0). Below the injection break energy ε 0, ion and electron energy spectra are extremely hard (p < ≲ 1) for any B g /B 0 and σ 0 ≳ 1, while above ε 0 the spectral index steepens (p > ≳ 2), displaying high sensitivity to both B g /B 0 and σ 0. The pitch angle displays power-law ranges with negative slopes (m <) below and positive slopes (m >) above ε min α , steepening with increasing B g /B 0 and σ 0. The ratio B g /B 0 regulates the redistribution of magnetic energy between ions (ΔE i ) and electrons (ΔE e ), with ΔE i ≫ ΔE e for B g /B 0 ≪ 1, ΔE i ∼ ΔE e for B g /B 0 ∼ 1, and ΔE i ≪ ΔE e for B g /B 0 ≫ 1, with ΔE i /ΔE e approaching unity when σ 0 ≫ 1. The anisotropic distribution of accelerated particles results in an optically thin synchrotron power spectrum F ν (ν) ∝ ν (2−2p+m)/(4+m) and a linear polarization degree Πlin = (p + 1)/(p + 7/3 + m/3) for a uniform magnetic field. Pitch-angle anisotropy also induces temperature anisotropy and eases synchrotron cooling, along with producing beamed radiation aligned with the magnetic field, which is potentially responsible for rapid frequency-dependent variability.