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 a total magnetization of ${\sigma }_0=B^2/\left(4\pi n\right(m_i+m_e\left)c^2\right)$, the magnetization for each species is ${\sigma }_i\sim {\sigma }_0$ and ${\sigma }_e\sim \left(m_i/m_e\right){\sigma }_0$, respectively. We have studied the magnetically dominated regime by varying σ_e = 10³–10⁵ with initial ion and electron temperatures $T_i=T_e=5-20m_e{c}^2$ 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 nonthermal high-energy emissions.