Coronal energy release by MHD avalanches

Abstract

Context. A possible key element for large-scale energy release in the solar corona is a magnetohydrodynamic (MHD) kink instability in a single twisted magnetic flux tube. An initial helical current sheet progressively fragments in a turbulent way into smaller-scale sheets. Dissipation of these sheets is similar to a nanoflare storm. Since the loop expands in the radial direction during the relaxation process, an unstable loop can disrupt nearby stable loops and trigger an MHD avalanche. Aims. Exploratory investigations have been conducted in previous works with relatively simplified loop configurations. In this work, we address a more realistic environment that comprehensively accounts for most of the physical effects involved in a stratified atmosphere typical of an active region. The questions we investigate are whether the avalanche process will be triggered, with what timescales, and how will it develop as compared with the original, simpler approach. Methods. We used three-dimensional MHD simulations to describe the interaction of magnetic flux tubes, which have a stratified atmosphere with chromospheric layers, a thin transition region to the corona, and a related transition from high-β to dlow-β regions. The model also includes the effects of thermal conduction and of optically thin radiation. Results. Our simulations address the case where one flux tube amongst a few is twisted at the footpoints faster than its neighbours. We show that this flux tube becomes kink unstable first in conditions in agreement with those predicted by analytical models. It then rapidly affects nearby stable tubes, instigating significant magnetic reconnection and dissipation of energy as heat. In turn, the heating brings about chromospheric evaporation as the temperature rises up to about 107 K, close to microflare observations. Conclusions. This work confirms, in more realistic conditions, that avalanches are a viable mechanism for the storing and release of magnetic energy in plasma confined in closed coronal loops as a result of photospheric motions.