Evidence for departure from a power-law flare size distribution for a small solar active region

Abstract

Active region 11029 was a small, highly flare-productive solar active region observed at a time of extremely low solar activity. The region produced only small flares: the largest of the >70 Geostationary Observational Environmental Satellite (GOES) events for the region has a peak 1-8 Å flux of 2.2 × 10-6 W m-2 (GOES C2.2). The background-subtracted GOES peak-flux distribution suggests departure from power-law behavior above 10-6 W m-2, and a Bayesian model comparison strongly favors a power-law plus rollover model for the distribution over a simple power-law model. The departure from the power law is attributed to this small active region having a finite amount of energy. The rate of flaring in the region varies with time, becoming very high for 2 days coinciding with the onset of an increase in complexity of the photospheric magnetic field. The observed waiting-time distribution for events is consistent with a piecewise-constant Poisson model. These results present challenges for models of flare statistics and of energy balance in solar active regions. © 2010. The American Astronomical Society.