Statistical Analysis of Circular-ribbon Flares

Circular-ribbon flares (CFs) are a special type of solar flares owing to their particular magnetic topology. In this paper, we conducted a comprehensive statistical analysis of 134 CFs from 2011 September to 2017 June, including 4 B-class, 82 C-class, 40 M-class, and 8 X-class flares. The flares were observed by the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory spacecraft. The physical properties of CFs are derived, including the location, area (A (CF)), equivalent radius (r (CF)) assuming a semispherical fan dome, lifetime (tau (CF)), and peak soft X-ray (SXR) flux in 1-8 angstrom. It is found that all CFs are located in active regions, with the latitudes between -30 degrees and 30 degrees. The distributions of areas and lifetimes could be fitted with a lognormal function. There is a positive correlation between the lifetime and area. The peak SXR flux in 1-8 angstrom is well in accord with a power-law distribution with an index of -1.42. For the 134 CFs, 57% of them are accompanied by remote brightenings or ribbons. A positive correlation exists between the total length (L (RB)) and average distance (D (RB)) of remote brightenings. About 47% and 51% of the 134 CFs are related to type III radio bursts and jets, respectively. The association rates are independent of flare energies. About 38% of CFs are related to minifilament eruptions, and the association rates increase with flare classes. Only 28% of CFs are related to coronal mass ejections (CMEs), meaning that a majority of them are confined rather than eruptive events. There is a positive correlation between the CME speed and peak SXR flux in 1-8 angstrom, and faster CMEs tend to be wider.