Concurrent Particle Acceleration and Pitch-Angle Anisotropy Driven by Magnetic Reconnection: Ion-Electron Plasmas

Luca Comisso

doi:10.3847/1538-4357/ad51fe

paper

Concurrent Particle Acceleration and Pitch-Angle Anisotropy Driven by Magnetic Reconnection: Ion-Electron Plasmas

arXiv:2405.18227 · doi:10.3847/1538-4357/ad51fe

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 $f_{s} (ε) \propto ε^{- p}$ and pitch angle $⟨ sin^{2} α ⟩ \propto ε^{m}$ . Their characteristics are determined by the ratio of the guide field to the reconnecting field ( $B_{g} / B_{0}$ ) and the plasma magnetization ( $σ_{0}$ ). Below the 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 $ε_{m i n α}$ , 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 $B_{g} / B_{0} ≪ 1$ , $Δ E_{i} \sim Δ E_{e}$ for $B_{g} / B_{0} \sim 1$ , 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 $F_{ν} (ν) \propto ν^{(2 - 2 p + m) / (4 + m)}$ and a linear polarization degree $Π_{lin} = (p + 1) / (p + 7/3 + m /3)$ . Pitch-angle anisotropy also induces temperature anisotropy and eases synchrotron cooling, along with producing beamed radiation, potentially responsible for frequency-dependent variability.

To appear in The Astrophysical Journal

Concurrent Particle Acceleration and Pitch-Angle Anisotropy Driven by Magnetic Reconnection: Ion-Electron Plasmas

Abstract

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