Thermal and non-thermal emission from reconnecting twisted coronal loops

Abstract

Context. Twisted magnetic fields should be ubiquitous in the solar corona, particularly in flare-producing active regions where the magnetic fields are strongly non-potential. The magnetic energy contained in such twisted fields can be released during solar flares and other explosive phenomena. It has been shown recently that reconnection in helical magnetic coronal loops results in plasma heating and particle acceleration distributed within a large volume, including the lower coronal and chromospheric sections of the loops. Hence, the magnetic reconnection and particle acceleration scenario involving magnetic helicity can be a viable alternative to the standard flare model, where particles are accelerated only in a small volume located in the upper corona. Aims. The key goal of this study is to investigate the links and observational signatures of plasma heating and particle acceleration in kink-unstable twisted coronal loops. Methods. We use a combination of MHD simulations and test-particle methods. These simulations describe the development of kink instability and magnetic reconnection in twisted coronal loops using resistive compressible MHD, and incorporate atmospheric stratification and large-scale loop curvature. The resulting distributions of hot plasma let us estimate thermal X-ray emission intensities. The electric and magnetic fields obtained are used to calculate electron trajectories using the guiding-centre approximation. These trajectories combined with the MHD plasma density distributions let us deduce synthetic hard X-ray bremsstrahlung intensities. Results. Our simulations emphasise that the geometry of the emission patterns produced by hot plasma in flaring twisted coronal loops can differ from the actual geometry of the underlying magnetic fields. In particular, the twist angles revealed by the emission threads (soft X-ray thermal emission; SXR) are consistently lower than the field-line twist present at the onset of the kink-instability. Hard X-ray (HXR) emission due to the interaction of energetic electrons with the stratified background are concentrated at the loop foot-points in these simulations, even though the electrons are accelerated everywhere within the coronal volume of the loop. The maximum of HXR emission consistently precedes that of SXR emission, with the HXR light-curve being approximately proportional to the temporal derivative of the SXR light-curve.

Plasma motions and non-thermal line broadening in flaring twisted coronal loops

“…The magnetic reconnection in twisted coronal loops atmosphere after kink instability has been described in many earlier studies (see Hood & Priest 1979;Baty & Heyvaerts 1996;Browning & Van der Linden 2003;Hood et al 2009;Gordovskyy & Browning 2012;Bareford et al 2016), while the evolution of thermal and non-thermal radiation from such systems is considered by Botha et al (2012), Gordovskyy et al (2013, Pinto et al (2016). Essentially, a loop experiences two different types of reconnection simultaneously: the reconnection between field lines of twisted fluxtube, resulting in twist reduction, and the reconnection of twisted field lines with the ambient field, resulting in the radial expansion of twisted loops (Fig.…”

“…The plasma temperature and, hence, thermal emission intensities peak towards the end of fast reconnection, when the twist is considerably lower. As the result, the visible twist in EUV/SXR is much lower than the critical twist required for the kink instability (Pinto et al 2016). …”

“…The simulations are based on the resistive singlefluid MHD, also incorporating Braginskii thermal conduction (Braginskii 1965) and radiative losses following Klimchuk et al (2008). Pre-kink configurations have been derived as described in Gordovskyy et al (2013), Bareford et al (2016), Pinto et al (2016). Although the initial atmosphere is in the hydrodynamical equilibrium, it is not stationary because of the thermal conduction.…”

“…The heat flux from the hot corona to colder chromosphere results in noticeable changes of the temperature around the transition region. Moderate heating of the transition region and chromosphere and moderate cooling of the lower corona lead to smoother temperature profiles near the transition region (see Pinto et al 2016;Bareford et al 2016). The atmosphere settles in about ∼1000 t A and only after this the initially potential loop is twisted by footpoint rotation.…”

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

“…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.…”