MHD avalanches involve small, intensely localized instabilities that
spread across neighbouring regions in a magnetic field. Cumulatively,
many small events release vast amounts of stored magnetic energy.
Straight cylindrical flux tubes, in Parker (1972)’s model of coronal
loops, are liable to such avalanches: one unstable flux tube can cause
instability to proliferate through reconnection, resulting in an ongoing
chain of like events. True coronal loops are curved, arching between
different footpoints on one photospheric plane. Using three-dimensional
MHD simulations, we here verify the viability of MHD avalanches within
the curved magnetic geometry of a multi-threaded coronal arcade. In
contrast to the behaviour of straight cylindrical models, a modified
ideal MHD kink mode occurs more readily and preferentially upwards in
this new geometry. Such instability spreads over a region far wider than
the original flux tubes, and wider than their photospheric footpoints.
Consequently, substantial and sustained heating is produced, in a series
of nanoflare-type events, contributing significantly to coronal heating.
Overwhelmingly dominant is viscous heating, attributable to the shocks
and jets produced around these small events. Reconnection is not the
greatest contributor to heating, but rather the facilitator of those
processes that are. Localized and intermittent, the heating shows no
strong spatial preference, except for a small bias away from footpoints.
Effects of realistic physical plasma parameters and the implications for
thermodynamic models, with energetic transport, are discussed.