Energy Release in Driven Twisted Coronal Loops

Abstract

We investigate magnetic reconnection in twisted magnetic fluxtubes, representing coronal loops. The main goal is to establish the influence of the field geometry and various thermodynamic effects on the stability of twisted fluxtubes and on the size and distribution of heated regions. In particular, we aim to investigate to what extent the earlier idealised models, based on the initially cylindrically symmetric fluxtubes, are different from more realistic models, including the large-scale curvature, atmospheric stratification, thermal conduction and other effects. In addition, we compare the roles of Ohmic heating and shock heating in energy conversion during magnetic reconnection in twisted loops. The models with straight fluxtubes show similar distribution of heated plasma during the reconnection: it initially forms a helical shape, which subsequently becomes very fragmented. The heating in these models is rather uniformly distributed along fluxtubes. At the same time, the hot plasma regions in curved loops are asymmetric and concentrated close to the loop tops. Large-scale curvature has a destabilising influence: less twist is needed for instability. Footpoint convergence normally delays the instability slightly, although in some cases, converging fluxtubes can be less stable. Finally, introducing a stratified atmosphere gives rise to decaying wave propagation, which has a destabilising effect.

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

“…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 kink instability occurs when the total twist reaches 6π−8π (see also Bareford et al 2016, for details). 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).…”

“…Secondly, the magnetic energy can be converted into kinetic energy of plasma turbulence well away from the reconnection regions. This is possible, for instance, if the plasma is heated and turbulised by MHD shocks generated by the magnetic reconnection, see Bareford & Hood (2015), Bareford et al (2016).…”

3d MHD Modeling of Twisted Coronal Loops

“…The stress of a magnetic flux tube has been studied due to twisting (e.g., Golub et al 1980;Baty 2000;Klimchuk et al 2000;Török & Kliem 2003) and braiding of the field lines (López Fuentes & Klimchuk 2010;Bingert & Peter 2011;Wilmot-Smith et al 2011). Most efforts have been devoted to studying the conditions and effects of the resulting kink instability (Hood & Priest 1979b;Zaidman & Tajima 1989;Velli et al 1990;Baty 2000;Gerrard et al 2001;Török et al 2004;Bareford et al 2016), and to the resulting formation of current sheets (Velli et al 1997;Kliem et al 2004) and relaxation due to several dissipation mechanisms (Hood et al 2009;Bareford et al 2013). MHD simulations have shown the possible importance of local instabilities in the coronal magnetic field to trigger cascades to large-scale energy release .…”

Flare particle acceleration in the interaction of twisted coronal flux ropes

“…Subsequent build-up and dissipation of secondary current sheet structures in later stages within the same tube continue to heat the corona in a manner consistent with the nanoflare picture (Browning et al 2008;Hood et al 2009), until the configuration approaches a minimal energy, helicity conserving Taylor state (Taylor 1974;Browning & Van der Linden 2003;Bareford et al 2013). This relatively simple initial concept has been further extended to incorporate additional effects, including atmospheric stratification, curvature, thermal conduction and others (Bareford et al 2016), while producing plasma motions which qualitatively and quantitatively agree with observations .…”