Rates of flaring in individual active regions

Rates of flaring in individual active regions on the Sun during the period 1981-1999 are examined using United States Air Force/Mount Wilson (USAF/MWL ) active-region observations together with the Geostationary Operational En vironmental Satellite (GOES) soft X-ray flare catalog. Of the flares in the catalog above C1 class, 61.5% are identified with an active region. Eviden ce is presented for obscuration, i.e. that the increase in soft X-ray flux during a large flare decreases the likelihood of detection of soft X-ray ev ents immediately following the large flare. This effect means that many eve nts are missing from the GOES catalog. It is estimated that in the absence of obscuration the number of flares above C1 class would be higher by (75 /- 23) %. A second observational selection effect - an increased tendency f or larger flares to be identified with an active region - is also identifie d. The distributions of numbers of flares produced by individual active reg ions and of mean flaring rate among active regions are shown to be approxim ately exponential, although there are excess numbers of active regions with low flare numbers and low flaring rates. A Bayesian procedure is used to a nalyze the time history of the flaring rate in the individual active region s. A substantial number of active regions appear to exhibit variation in fl aring rate during their transit of the solar disk. Examples are shown of re gions with and without rate variation, illustrating the different distribut ions of times between events (waiting-time distributions) that are observed . A piecewise constant Poisson process is found to provide a good model for the observed waiting-time distributions. Finally, applications of analysis of the rate of flaring to understanding the flare mechanism and to flare p rediction are discussed.