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 $f_s\left(|\cos \hat{I}\pm |,\epsilon \right)$, characterized by broken power laws in the particle energy spectrum $f_s(\epsilon )\propto {\epsilon }^{-p}$ and pitch angle $⟨{\sin }^2\hat{I}\pm ⟩\propto {\epsilon }^m$. Their characteristics are determined by the ratio of the guide field to the reconnecting field ($B_g/B_0$) and the plasma magnetization ($\ddot{I}{ƒ}_0$). Below the break energy ${\epsilon }_0$, ion and electron energy spectra are extremely hard ($p_{<}\lesssim 1$) for any $B_g/B_0$ and $\ddot{I}{ƒ}_0\gtrsim 1$, while above ${\epsilon }_0$, the spectral index steepens ($p_{>}\gtrsim 2$), displaying high sensitivity to both $B_g/B_0$ and $\ddot{I}{ƒ}_0$. The pitch angle displays power-law ranges with negative slopes ($m_{<}$) below and positive slopes ($m_{>}$) above ${\epsilon }_{\min \hat{I}\pm }$, steepening with increasing $B_g/B_0$ and $\ddot{I}{ƒ}_0$. The ratio $B_g/B_0$ regulates the redistribution of magnetic energy between ions ($\hat{I}”E_i$) and electrons ($\hat{I}”E_e$), with $\hat{I}”E_i\gg \hat{I}”E_e$ for $B_g/B_0\ll 1$, $\hat{I}”E_i\sim \hat{I}”E_e$ for $B_g/B_0\sim 1$, and $\hat{I}”E_i\ll \hat{I}”E_e$ for $B_g/B_0\gg 1$, with $\hat{I}”E_i/\hat{I}”E_e$ approaching unity when $\ddot{I}{ƒ}_0\gg 1$. The anisotropic distribution of accelerated particles results in an optically thin synchrotron power spectrum $F_{\hat{I}}½(\hat{I}½)\propto \hat{I}{½}^{(2-2p+m)/(4+m)}$ and a linear polarization degree ${\hat{I}}_{\mathrm{lin}}=(p+1)/(p+7/3+m/3)$. 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