Test particle sampling and particle acceleration in a 2D coronal plasmoid-mediated reconnecting current sheet

¹ Rosseland Centre for Solar Physics, University of Oslo, Sem Sælands vei 13, Oslo, Norway
² Institute of Theoretical Astrophysics, University of Oslo, Sem Sælands vei 13, Oslo, Norway
³ Scottish Universities Physics Alliance, School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, United Kingdom

^⋆ Corresponding author: e.s.oyre@astro.uio.no

Received: 7 May 2025
Accepted: 29 August 2025

Abstract

Context. Solar flares accelerate electrons, creating non-thermal energy distributions. However, the acceleration sites and dominant acceleration mechanisms remain largely unknown.

Aims. We study the characteristics of electron acceleration and subsequent non-thermal energy distribution in a 2D coronal plasmoid-mediated reconnecting current sheet.

Methods. We used test particles and the guiding centre approximation to transport electrons in a static coronal 2D fan-spine topology magnetohydrodynamic (MHD) snapshot. The snapshot was from a Bifrost simulation that featured plasmoid-mediated reconnection at a current sheet. To sample initial particle conditions that lead to non-thermal energies, we used importance sampling. In this way, the characteristics of the non-thermal electrons were statistically representative of the MHD plasma.

Results. The energy distribution of the electrons forms a non-thermal power law that varies with our tolerance of the guiding centre approximation’s validity, from no obvious power law to a power law with an exponent of −4 (the power law also depends on the statistical weighing of the electrons). The non-thermal electrons gain energy through a gradual betatron acceleration close to magnetic null points associated with plasmoids.

Conclusions. In this static, asymmetric, coronal, 2D fan-spine topology MHD configuration, non-thermal electron acceleration occurs only in the vicinity of null points associated with magnetic gradients and electric fields induced by plasmoid formation and ejection. However, the guiding centre approximation alone is not sufficient to properly estimate the shape of the non-thermal power law since, according to our results, electron acceleration is correlated with the adiabaticity of the particles’ motion. The results also show that the particle power law formation is biased by the test particle sampling procedure.