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

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 (Δ E i ) and electrons (Δ E e ), with Δ E i ≫ Δ E e for B g / B 0 ≪ 1, Δ E i ∼ Δ E e for B g / B 0 ∼ 1, and Δ E i ≪ Δ E e for B g / B 0 ≫ 1, with Δ E i /Δ E 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 (2−2 p + m )/(4+ m ) and a linear polarization degree Π lin = ( 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.