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

TL;DR

This study uses kinetic simulations to explore how magnetic reconnection accelerates particles and creates pitch-angle anisotropy in ion-electron plasmas, revealing how these effects depend on magnetic field ratios and plasma magnetization.

Contribution

It provides new insights into particle energy spectra, pitch-angle distributions, and energy redistribution during magnetic reconnection in ion-electron plasmas, with detailed dependencies on key plasma parameters.

Findings

Ion and electron spectra are extremely hard below the break energy.

Spectral index steepens above the break energy, sensitive to guide field and magnetization.

Pitch angle anisotropy influences synchrotron emission and variability.

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 $f_s (\varepsilon) \propto \varepsilon^{-p}$ and pitch angle $\langle \sin^2 \alpha \rangle \propto \varepsilon^m$. Their characteristics are determined by the ratio of the guide field to the reconnecting field ($B_g/B_0$) and the plasma magnetization ($\sigma_0$). Below the break energy $\varepsilon_0$, ion and electron energy spectra are extremely hard ($p_<\lesssim 1$) for any $B_g/B_0$ and $\sigma_0 \gtrsim 1$, while above $\varepsilon_0$, the spectral index steepens ($p_> \gtrsim 2$), displaying high sensitivity to both $B_g/B_0$ and $\sigma_0$. The pitch angle displays power-law ranges with negative slopes ($m_<$) below and positive slopes ($m_>$) above $\varepsilon_{\min \alpha}$, steepening with increasing $B_g/B_0$ and $\sigma_0$. The ratio $B_g/B_0$ regulates the redistribution of magnetic energy between ions ($\Delta E_i$) and electrons ($\Delta E_e$), with $\Delta E_i \gg \Delta E_e$ for $B_g/B_0 \ll 1$, $\Delta E_i \sim \Delta E_e$ for $B_g/B_0 \sim 1$, and $\Delta E_i \ll \Delta E_e$ for $B_g/B_0 \gg 1$, with $\Delta E_i/\Delta E_e$ approaching unity when $\sigma_0 \gg 1$. The anisotropic distribution of accelerated particles results in an optically thin synchrotron power spectrum $F_\nu(\nu) \propto\nu^{(2-2p+m)/(4+m)}$ and a linear polarization degree $\Pi_{\rm lin} = (p+1)/(p+7/3+m/3)$. Pitch-angle anisotropy also induces temperature anisotropy and eases synchrotron cooling, along with producing beamed radiation, potentially responsible for frequency-dependent variability.