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A possible key element for large-scale energy release in the solar corona isan MHD kink instability in a single twisted magnetic flux tube. An initialhelical current sheet fragments in a turbulent way into smaller-scale sheets,similarly to a nanoflare storm. As the loop expands in the radial directionduring the relaxation process, an unstable loop can disrupt nearby stable loopsand trigger an MHD avalanche. Exploratory investigations have been conducted inprevious works with relatively simplified loop configurations. Here, we addressa more realistic environment that comprehensively accounts for most of thephysical effects involved in a stratified atmosphere, typical of an activeregion. The question is whether the avalanche process will be triggered, withwhat timescales, and how it will develop, as compared with the original,simpler approach. Three-dimensional MHD simulations describe the interaction ofmagnetic flux tubes, which have a stratified atmosphere, includingchromospheric layers, the thin transition region to the corona, and the relatedtransition from high-beta to low-beta regions. The model also includes theeffects of thermal conduction and of optically thin radiation. Our simulationsaddress the case where one flux tube among a few is twisted at the footpointsfaster than its neighbours. We show that this flux tube becomes kink unstablefirst, in conditions in agreement with those predicted by analytical models. Itrapidly involves nearby stable tubes, instigating significant magneticreconnection and dissipation of energy as heat. The heating determines thedevelopment of chromospheric evaporation, while the temperature rises up toabout 10 MK, close to microflares observations. This work confirms thatavalanches are a viable mechanism for the storing and release of magneticenergy in plasma confined in closed coronal loops, as a result of photosphericmotions.

Coronal energy release by MHD avalanches. Effects on a structured, active region, multi-threaded coronal loop