Power Laws in Solar Flares: Self-Organized Criticality or Turbulence?

The statistics of quiescent times ${\tau }_L$ between successive bursts of solar flares activity, performed using 20 years of data, displays a power law distribution with exponent $\alpha \simeq 2.4$. This is an indication of an underlying complex dynamics with long correlation times. The observed scaling behavior is in contradiction with the self-organized criticality models of solar flares which predict Poisson-like statistics. Chaotic models, including the destabilization of the laminar phases and subsequent restabilization due to nonlinear dynamics, are able to reproduce the power law for the quiescent times. A shell model of MHD turbulence correctly reproduces all the observed distributions.