  A possible key element for large-scale energy release in the solar corona is
an MHD kink instability in a single twisted magnetic flux tube. An initial
helical current sheet fragments in a turbulent way into smaller-scale sheets,
similarly to a nanoflare storm. As the loop expands in the radial direction
during the relaxation process, an unstable loop can disrupt nearby stable loops
and trigger an MHD avalanche. Exploratory investigations have been conducted in
previous works with relatively simplified loop configurations. Here, 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 question is whether the avalanche process will be triggered, with
what timescales, and how it will develop, as compared with the original,
simpler approach. Three-dimensional MHD simulations describe the interaction of
magnetic flux tubes, which have a stratified atmosphere, including
chromospheric layers, the thin transition region to the corona, and the related
transition from high-beta to low-beta regions. The model also includes the
effects of thermal conduction and of optically thin radiation. Our simulations
address the case where one flux tube among 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
rapidly involves nearby stable tubes, instigating significant magnetic
reconnection and dissipation of energy as heat. The heating determines the
development of chromospheric evaporation, while the temperature rises up to
about 10 MK, close to microflares observations. This work confirms 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.
