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 $f_s (\varepsilon) \propto \varepsilon^{-p}$ 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.