Particle dynamics in a non-flaring solar active region model

¹ School of Mathematics and Statistics, University of St Andrews, St Andrews, Fife KY16 9SS, UK
e-mail: jwt9@st-andrews.ac.uk; tn3@st-andrews.ac.uk; cep@st-andrews.ac.uk
² Space Research Institute, Austrian Academy of Sciences, Schmiedlstr. 6, 8042 Graz, Austria
e-mail: Philippe.Bourdin@oeaw.ac.at

Received: 2 June 2015
Accepted: 16 December 2015

Abstract

Aims. The aim of this work is to investigate and characterise particle behaviour in an (observationally-driven) 3D magnetohydrodynamic (MHD) model of the solar atmosphere above a slowly evolving, non-flaring active region.

Methods. 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.

Results. 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 (≲42 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.