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

The National Astronomical Observatories, the Chinese Academy of Sciences, A20, Datun Rd., ChaoYang District, Beijing, PR China e-mail: zhangliyun@bao.ac.cn

Received: 16 February 2007
Accepted: 14 May 2007

Abstract

Context.Recently, it was found that two persistent active longitudes of sunspots separated by about 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 , where Ω is the angular velocity at a given latitude φ, while is the equatorial angular velocity and B describes the differential rotation rate. Both and B are the parameters that need to be determined from observations.

Results.The authors found that there are two active longitudes separated by 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.