Large-scale Compression Acceleration during Magnetic Reconnection in a Low-β Plasma

Xiaocan Li,Fan Guo,Shengtai Li,Hui Li

doi:10.3847/1538-4357/aae07b

Abstract

Abstract In solar flares and other astrophysical systems, a major challenge for solving the particle acceleration problem associated with magnetic reconnection is the enormous scale separation between kinetic scales and the observed reconnection scale. Because of this, it has been difficult to draw any definite conclusions by just using kinetic simulations. A particle acceleration model that solves the energetic particle transport equation can capture the main acceleration physics found in kinetic simulations and thus provide a practical way to make observable predictions and directly compare model results with observations. Here we study compression particle acceleration in magnetic reconnection by solving the Parker (diffusion–advection) transport equation using velocity and magnetic fields from two-dimensional magnetohydrodynamics (MHD) simulations of a low-β high-Lundquist-number reconnection layer. We show that the compressible reconnection layer can give significant particle acceleration, leading to the formation of power-law particle energy distributions. We analyze the acceleration rate and find that the acceleration in the reconnection layer is a mixture of first- and second-order Fermi processes. When including a guide field, we find that the spectrum becomes steeper and both the power-law cutoff energy and maximum particle energy decrease as plasma becomes less compressible. This model produces a 2D particle distribution that one can use to generate a radiation map and directly compare with solar flare observations. This provides a framework to explain particle acceleration at large-scale astrophysical reconnection sites, such as solar flares.

Highlights

Energy conversion and particle acceleration in strongly magnetized plasmas are important processes that hold the key for understanding many explosive solar and astrophysical high-energy phenomena (Zweibel & Yamada 2009; Lin 2011)
To examine the nature of particle acceleration in a reconnection layer, we study how the particle acceleration rate depends on the flow speed, which is about the Alfvén speed vA in a reconnection layer
We found that compression in the reconnection layer leads to significant particle acceleration and the formation of power-law energy distributions for both electrons and ions

Summary

Introduction

Energy conversion and particle acceleration in strongly magnetized plasmas are important processes that hold the key for understanding many explosive solar and astrophysical high-energy phenomena (Zweibel & Yamada 2009; Lin 2011). A large amount of electrons in the flare region (> 1036 electrons) are accelerated into a power-law energy spectrum f (ε) ∝ ε−s with spectral index from s ∼ 3 to more than s = 9 with a medium about 5 (Lin & Hudson 1976; Krucker et al 2010; Oka et al 2013, 2015; Effenberger et al 2017). In-situ solar energetic particle (SEP) observation has shown that the electron and ion spectra often resemble power-law distributions (Mason et al 2012). How such efficient particle acceleration occurs over a large-scale reconnection region remains an important unsolved problem in reconnection study

Objectives

Methods

Results

Conclusion