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-2/4 pi n(m(i) + m(e))c(2)), the magnetization for each species is sigma(i) similar to sigma(0) and sigma(e) similar to(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 - 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) similar to sigma(e) and gamma(bi) similar to 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.