Particle acceleration and transport in reconnecting twisted loops in a stratified atmosphere

Abstract

Context. Twisted coronal loops should be ubiquitous in the solar corona. Twisted magnetic fields contain excess magnetic energy, which can be released during magnetic reconnection, causing solar flares. Aims. The aim of this work is to investigate magnetic reconnection, and particle acceleration and transport in kink-unstable twisted coronal loops, with a focus on the effects of resistivity, loop geometry and atmospheric stratification. Another aim is to perform forward-modelling of bremsstrahlung emission and determine the structure of hard X-ray sources. Methods. We use a combination of magnetohydrodynamic (MHD) and test-particle methods. First, the evolution of the kinking coronal loop is considered using resistive MHD model, incorporating atmospheric stratification and loop curvature. Then, the obtained electric and magnetic fields and density distributions are used to calculate electron and proton trajectories using a guiding-centre approximation, taking into account Coulomb collisions. Results. It is shown that electric fields in twisted coronal loops can effectively accelerate protons and electrons to energies up to 10 MeV. High-energy particles have hard, nearly power-law energy spectra. The volume occupied by high-energy particles demonstrates radial expansion, which results in the expansion of the visible hard X-ray loop and a gradual increase in hard X-ray footpoint area. Synthesised hard X-ray emission reveals strong footpoint sources and the extended coronal source, whose intensity strongly depends on the coronal loop density.

Reconnection and particle acceleration in interacting flux ropes – II. 3D effects on test particles in magnetically dominated plasmas

“…This observation is visible in all simulations carried out in this work. The tendency for particle to accelerate along the magnetic field lines, and hence obtain a very small pitch angle, is in agreement with the findings of Gordovskyy et al (2014). However, curvature acceleration resulting in increasing parallel velocity is neglected in their setup and particle collisions are incorporated.…”

“…In this work, we treat electrons and ions as test particles embedded in a thermal (MHD) plasma. Particle acceleration associated with reconnection and shocks in magnetically dominated Newtonian plasmas in the solar corona is studied extensively with test particle approaches (Rosdahl & Galsgaard 2009, Gordovskyy et al 2010, Gordovskyy & Browning 2011a, Gordovskyy & Browning 2011b, Gordovskyy et al 2014, Zhou et al 2015, Pinto et al 2016, Zhou et al 2016, Ripperda et al 2017, Threlfall et al 2017 and even in relativistic plasmas in the context of pulsar wind nebulae (Porth et al 2016, Giacchè & Kirk 2017. The test particles are guided by MHD fields without giving feedback to these fields.…”

Particle Acceleration Due to Coronal Non-null Magnetic Reconnection

“…A similar, more recent model of reconnecting coronal loops (Gordovskyy et al, 2014) also shows high-energy proton and electron precipitation towards the loop footpoints. These authors recover a high-energy power-law tail using initially Maxwellian particle energies, likely due to the reconnection of multiple fragmented current sheets, again implying that our model lacks the appropriate size and sporadic nature of the electric field using the parameter values specified here.…”

“…Threlfall et al, 2015Threlfall et al, , 2016aBorissov, Neukirch, and Threlfall, 2016), with similar implementations in other recent investigations (e.g. Gordovskyy et al, 2014). By controlling the timescales of the guiding-centre approximation code and the timescale of the global flux-rope eruption model so that they never become of a similar size, we are justified in our use of this test-particle approach.…”

“…The application of test-particle analysis to simulations of larger structures embedded in the solar atmosphere, including coronal loops (e.g. Gordovskyy et al, 2014) or indeed entire active regions (Threlfall et al, 2016b) has also uncovered evidence of significant particle acceleration. It is noteworthy that such large-scale structures often contain many locations and topological features (such as nulls, separatrix surfaces, spine lines, and separators) where reconnection takes place.…”