Investigation of White-light Emission in Circular-ribbon Flares.

Using observations by the Solar Dynamics Observatory from 2010 June to 2017 December, we have performed the first statistical investigation of circular-ribbon flares (CFs) and examined the white-light emission in them. We find 90 CFs occurring in 36 active regions (ARs), including eight X-class, 34 M-class, and 48 C- and B-class flares. The occurrence rate of white-light flares (WLFs) is 100% (8/8) for X-class CFs, ∼62% (21/34) for M-class CFs, and ∼8% (4/48) for C- and B-class CFs. Sometimes we observe several CFs in a single AR, and nearly all of them are WLFs. Compared to normal CFs, those with white-light enhancement tend to have a shorter duration, smaller size, stronger electric current and more complicated magnetic field. We find that for X-class WLFs, the white-light enhancement is positively correlated with the flare class, implying that it is largely determined by the amount of released energy. However, there is no such correlation for M- and C-class WLFs, suggesting that other factors such as the timescale, spatial scale, and magnetic field complexity may play important roles in the generation of white-light emission if the released energy is not high enough. [ABSTRACT FROM AUTHOR]