Charged-particle acceleration in a reconnecting current sheet including multiple magnetic islands and a nonuniform background magnetic field

¹ Yunnan Observatories, Chinese Academy of Sciences, Kunming, 650216 Yunnan, PR China
e-mail: liyan821@ynao.ac.cn
² Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences, 100012 Beijing, PR China
³ Center for Astronomical Mega-Science, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, 100012 Beijing, P.R. China
⁴ School of Tourism and Geography, Yunnan Normal University, 650031 Kunming, PR China

Received: 8 November 2016
Accepted: 14 June 2017

Abstract

Context. Charged particles are accelerated to high energies in solar flares. Although we know that magnetic reconnection is an efficient mechanism in generating energetic particles, the detailed role it plays in accelerating particles is still unknown.

Aims. We investigate particle acceleration by magnetic reconnection in the current sheet, including multiple islands and a guide field.

Methods. The long current sheet produced by the disruption in the corona magnetic field is usually not stable to various plasma instabilities, among which the tearing mode is the most important, and magnetic islands start to form in the current sheet when these instabilities develop. Two reverse processes are typically observed in the sheet: cascading of large islands to smaller ones, and merging of small islands into larger ones. Coalescent reconnection consequently takes place between two adjacent islands when merging occurs. The electric field induced by the coalescent reconnection is opposite to the electric field of the primary large-scale reconnection. We studied particle acceleration in such a current sheet and examined in detail the dynamic properties of electrons and protons in the current sheet through test particle approach.

Results. We found that some particles can be accelerated to high energies in a very short time, and some particles (near the coalescence reconnection site) are accelerated and decelerated back and forth by the primary and secondary electric fields. Particle motions show two distinct types along different trajectories: some particles are trapped around magnetic islands, and some escape from the current sheet mainly along open field lines. With the presence of a guide field, protons and electrons are found to eventually move in different directions. The energy spectra for both species follow a double power-law shape. The softer components of the power-law spectrum are due to the particles that are trapped and circulate around magnetic islands, while the particles that escape and are partly trapped contribute to the harder component of the spectrum. Finally, we also investigated the influences of the parameters for the reconnection process on the spectral feature.