Signature of an avalanche in solar flares as measured by photospheric magnetic fields

We analyzed time variations of turbulent parameters of the photospheric magnetic field of four active regions obtained during the course of major solar flares using longitudinal magnetograms from the Big Bear Solar Observatory and from SOHO/MDI full-disk measurements. Analysis of the data indicated that, before each flare, the degree of intermittency of the magnetic field had been increasing for 6-33 minutes and reached a maximum value approximately 3-14 minutes before the peak of the hard X-ray emission for each event. This result seems to suggest the existence in an active region of a turbulent phase prior to a solar flare. We also found that the maximum of the correlation length of the magnetic energy dissipation field tends to follow (or to occur nearly simultaneously) with the peak of the hard X-ray emission. The data suggest that the peak in the correlation length might be a trace of an avalanche of coronal reconnection events. We discuss the results in the framework of the concept of self-organized criticality.