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 ( τ CF ), and peak soft X-ray (SXR) flux in 1–8 Å. It is found that all CFs are located in active regions, with the latitudes between −30° and 30°. 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 Å 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 Å, and faster CMEs tend to be wider.