Long-term behavior of active longitudes for solar X-ray flares

Abstract

Context. Recently, it was found that two persistent active longitudes of sunspots separated by about 180° existed on the sun by considering a dynamic reference frame inferred from the differential rotation law. The same phenomenon was found in the longitudinal distribution of powerful X-ray flares. Aims. To study the statistical characteristics of active longitudes of solar activities, we analyzed the data sets of optical flares associated with all the solar X-ray flares > C class observed by GOES during the period of 1975 to 2005, of sunspots collected at the Royal Greenwich Observatory, the US Air Force, and NOAA for the same period as X-ray flares. Methods. The differential rotation law on the Sun is simply described as Ω = Ω 0 - B sin 2 φ , where Ω is the angular velocity at a given latitude φ, while Ω 0 is the equatorial angular velocity and B describes the differential rotation rate. Both Ω 0 and B are the parameters that need to be determined from observations. Results. The authors found that there are two active longitudes separated by 180° for X-ray flares of any class, which have existed for tens of years, that X-ray flares occur more often near the two active longitudes than sunspots do, and that the non-axisymmetry of the longitudinal distribution of X-ray flares increases with the X-ray flare class. Conclusions. Stronger solar activities occur more preferentially at certain longitudes.