Shock heating in numerical simulations of kink-unstable coronal loops

Abstract

An analysis of the importance of shock heating within coronal magnetic fields has hitherto been a neglected area of study. We present new results obtained from nonlinear magnetohydrodynamic simulations of straight coronal loops. This work shows how the energy released from the magnetic field, following an ideal instability, can be converted into thermal energy, thereby heating the solar corona. Fast dissipation of magnetic energy is necessary for coronal heating and this requirement is compatible with the time scales associated with ideal instabilities. Therefore, we choose an initial loop configuration that is susceptible to the fast-growing kink, an instability that is likely to be created by convectively driven vortices, occurring where the loop field intersects the photosphere (i.e. the loop footpoints). The largescale deformation of the field caused by the kinking creates the conditions for the formation of strong current sheets and magnetic reconnection, which have previously been considered as sites of heating, under the assumption of an enhanced resistivity. However, our simulations indicate that slow mode shocks are the primary heating mechanism, since, as well as creating current sheets, magnetic reconnection also generates plasma flows that are faster than the slow magnetoacoustic wave speed.

The effect of anisotropic viscosity on the nonlinear MHD kink instability

“…As a first step of investigating the effects of anisotropic viscosity on coronal dynamics, we focus on the kink instability [18,19], believed to be a trigger for flares [20] and an important mechanism in the theory of coronal heating through nanoflares [21]. The instability has also been studied using shock viscosity [19,22] but a detailed investigation of the effects of Newtonian and Braginskii viscosity has not, to the best of our knowledge, been performed. In particular, the main aim of this paper is to provide insight into the effect of the choice of viscosity model on the nonlinear dynamics and relaxation of a twisted coronal loop, where the kink instability converts magnetic energy to heat through Ohmic heating generated via current structures and through viscous heating generated via flow structures.…”

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

“…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).…”

Recent advances in coronal heating

“…Detailed models of the build-up of energy in magnetic fields and the subsequent energy release include models of field braiding (as reviewed by [22]) and relaxation following an ideal MHD instability in a twisted field [23]. The process by which reconnection actually dissipates the energy requires further investigation-dissipation need not take place mainly through Ohmic resistivity within current sheets, but rather in larger scale structures such as shocks [20,23].…”