Particle Acceleration during Magnetic Reconnection in a Low-beta Pair Plasma

Abstract

Plasma energization through magnetic reconnection in the magnetically-dominated regime featured by low plasma beta ($\beta = 8 \pi nkT_0/B^2 \ll 1$) and/or high magnetization ($\sigma = B^2/(4 \pi nmc^2) \gg 1$) is important in a series of astrophysical systems such as solar flares, pulsar wind nebula, and relativistic jets from black holes, etc. In this paper, we review the recent progress on kinetic simulations of this process and further discuss plasma dynamics and particle acceleration in a low-$\beta$ reconnection layer that consists of electron-positron pairs. We also examine the effect of different initial thermal temperatures on the resulting particle energy spectra. While earlier papers have concluded that the spectral index is smaller for higher $\sigma$, our simulations show that the spectral index approaches $p=1$ for sufficiently low plasma $\beta$, even if $\sigma \sim 1$. Since this predicted spectral index in the idealized limit is harder than most observations, it is important to consider effects that can lead to a softer spectrum such as open boundary simulations. We also remark that the effects of 3D reconnection physics and turbulence on reconnection need to be addressed in the future.