Efficient Production of High-energy Nonthermal Particles during Magnetic Reconnection in a Magnetically-dominated Ion-Electron Plasma

Magnetic reconnection is a leading mechanism for dissipating magnetic energy and accelerating nonthermal particles in Poynting-flux dominated flows. In this letter, we investigate nonthermal particle acceleration during magnetic reconnection in a magnetically-dominated ion-electron plasma using fully kinetic simulations. For an ion-electron plasma with the total magnetization $\sigma_0=B^2/(4\pi n(m_i+m_e)c^2)$, the magnetization for each species is $\sigma_i \sim \sigma_0$ and $\sigma_e \sim (m_i/m_e) \sigma_0$, respectively. We have studied the magnetically dominated regime by varying $\sigma_{e} = 10^3 - 10^5$ with initial ion and electron temperatures $T_i = T_e = 5 - 20 m_ec^2$ and mass ratio $m_i/m_e = 1 - 1836$. The results demonstrate that reconnection quickly establishes power-law energy distributions for both electrons and ions within several ($2-3$) light-crossing times. For the cases with periodic boundary conditions, the power-law index is $1<s<2$ for both electrons and ions. The hard spectra limit the power-law energies for electrons and ions to be $\gamma_{be} \sim \sigma_e$ and $\gamma_{bi} \sim \sigma_i$, respectively. The main acceleration mechanism is a Fermi-like acceleration through the drift motions of charged particles. When comparing the spectra for electrons and ions in momentum space, the spectral indices $s_p$ are identical as predicted in Fermi acceleration. We also find that the bulk flow can carry a significant amount of energy during the simulations. We discuss the implication of this study in the context of Poynting-flux dominated jets and pulsar winds especially the applications for explaining the nonthermal high-energy emissions.