The Astrophysical Journal (Jan 2024)
Concurrent Particle Acceleration and Pitch-angle Anisotropy Driven by Magnetic Reconnection: Ion-electron Plasmas
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 ${f}_{s}\left(| \cos \alpha | ,\varepsilon \right)$ , characterized by broken power laws in the particle energy spectrum f _s ( ε ) ∝ ε ^− ^p and pitch angle $\langle {\sin }^{2}\alpha \rangle \propto {\varepsilon }^{m}$ . 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 _ ≳ 2), displaying high sensitivity to both B _g / B _0 and σ _0 . The pitch angle displays power-law ranges with negative slopes ( m _ ) above ${\varepsilon }_{\min \alpha }$ , 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.
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