Particle dynamics in a non-flaring solar active region model

TL;DR

This study investigates particle behavior in a 3D MHD model of a non-flaring solar active region, revealing two main acceleration mechanisms and the presence of high-energy particle trapping, with implications for solar heating and reconnection.

Contribution

It introduces a detailed analysis of particle acceleration and trapping in a realistic 3D MHD simulation of a non-flaring active region, highlighting new acceleration behaviors and trapping phenomena.

Findings

Two distinct particle acceleration behaviors identified.

Particles can reach non-thermal energies up to 42 MeV.

High-energy trapping occurs independently of magnetic field strength.

Abstract

The aim of this work is to investigate and characterise particle behaviour in an (observationally-driven) 3D MHD model of the solar atmosphere above a slowly evolving, non-flaring active region. We use a relativistic guiding-centre particle code to investigate the behaviour of selected particle orbits, distributed throughout a single snapshot of the 3D MHD simulation. Two distinct particle acceleration behaviours are recovered, which affect both electrons and protons: (i) direct acceleration along field lines and (ii) tangential drifting of guiding centres with respect to local magnetic field. However, up to 40\% of all particles actually experience a form of (high energy) particle trap, because of changes in the direction of the electric field and unrelated to the strength of the magnetic field; such particles are included in the first category. Additionally, category (i) electron and proton orbits undergo surprisingly strong acceleration to non-thermal energies ($\lesssim42$MeV), because of the strength and extent of super-Dreicer electric fields created by the MHD simulation. Reducing the electric field strength of the MHD model does not significantly affect the efficiency of the (electric field-based) trapping mechanism, but does reduce the peak energies gained by orbits. We discuss the implications for future experiments, which aim to simulate non-flaring active region heating and reconnection.