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 fₛ ( |cos α|,ε ), characterized by broken power laws in the particle energy spectrum fₛ (ε) ∝ ε⁻ᵖ and pitch angle sin² α ∝ εᵐ. Their characteristics are determined by the ratio of the guide field to the reconnecting field (Bg/B₀) and the plasma magnetization (σ₀). Below the break energy ε₀, ion and electron energy spectra are extremely hard (p_< 1) for any Bg/B₀ and σ₀ 1, while above ε₀, the spectral index steepens (p_> 2), displaying high sensitivity to both Bg/B₀ and σ₀. The pitch angle displays power-law ranges with negative slopes (m_<) below and positive slopes (m_>) above εmin α, steepening with increasing Bg/B₀ and σ₀. The ratio Bg/B₀ regulates the redistribution of magnetic energy between ions (Δ Eᵢ) and electrons (Δ Eₑ), with Δ Eᵢ Δ Eₑ for Bg/B₀ 1, Δ Eᵢ ~ Δ Eₑ for Bg/B₀ ~ 1, and Δ Eᵢ Δ Eₑ for Bg/B₀ 1, with Δ Eᵢ/Δ Eₑ approaching unity when σ₀ 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). Pitch-angle anisotropy also induces temperature anisotropy and eases synchrotron cooling, along with producing beamed radiation, potentially responsible for frequency-dependent variability.