EFFICIENT PRODUCTION OF HIGH-ENERGY NONTHERMAL PARTICLES DURING MAGNETIC RECONNECTION IN A MAGNETICALLY DOMINATED ION-ELECTRON PLASMA

Abstract

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 σ<inf>0</inf> = B²/(4πn(m<inf>i</inf> + m<inf>e</inf>)c²), the magnetization for each species is σ<inf>i</inf> ∼ σ<inf>0</inf> and σ<inf>e</inf> ∼ (m<inf>i</inf>/m<inf>e</inf>)σ<inf>0</inf>, respectively. We have studied the magnetically dominated regime by varying σ<inf>e</inf> = 10³-10⁵ with initial ion and electron temperatures T<inf>i</inf> = T<inf>e</inf> = 5 - 20m<inf>e</inf>c² and mass ratio m<inf>i</inf>/m<inf>e</inf> = 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 γ<inf>be</inf> ∼ σ<inf>e</inf> and γ<inf>bi</inf> ∼ σ<inf>i</inf>, 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<inf>p</inf> 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.