AVALANCHES AND THE DISTRIBUTION OF SOLAR-FLARES

We propose that the solar coronal magnetic field is in a self-organized critical state, thus explaining the observed power-law dependence of solar flare occurrence rate on flare size which extends over more than five orders of magnitude in peak flux. The physical picture that arises is that solar flares are avalanches of many small reconnection events, analogous to avalanches of sand in the models published by Bak and colleagues in 1987 and 1988. Flares of all sizes are manifestations of the same physical processes, where the size of a given flare is determined by the number of elementary reconnection events. The relation between small-scale processes and the statistics of global flare properties which follows from the self-organized magnetic field configuration provides a way to learn about the physics of the unobservable small-scale reconnection processes. We present a simple lattice reconnection model which is consistent with the observed flare statistics. We discuss the implications for coronal heating and suggest some observational tests of this picture.