Detection and analysis of white-light emission in solar flares through light curve diagnostics

Context. White-light flares (WLFs) are crucial for understanding the energy transport and heating processes in the lower solar atmosphere. Systematic studies are highly necessary. However, most WLFs are very weak and difficult to detect. Aims. To address this, we propose a new method of detecting WLFs. Methods. Through the observations of SDO/HMI, the light curve of each pixel in the flaring region can be obtained. By subtracting the slowly varying background, we obtained a series of rapidly varying radiative pulses. Pixels for which radiative pulses during flares significantly exceed those occurring before and after the flare were identified as WL emission regions. Results. We applied our method to the detection of the X2.2 flare on September 6, 2017 and validated the method. We found that the WL emission in this flare exhibits two phases, and that different regions show distinct WL emission properties. We also detected the WL emission in all the flares (1 X-class, 2 M-class, and 20 C-class) occurred in active region NOAA 12887. It was found that 15 of the 23 flares are WLFs (1 X-class, 2 M-class, and 12 C-class). The occurrence rate of WLFs in this active region is ∼65%. Surprisingly, the occurrence rate of WLFs in C-class flares even reaches up to 60%. It should be noted that most of these C-class WLFs are below C5.0. In addition, a C1.0 WLF was identified; this is the lowest GOES-class event with confirmed WL emission to date. Conclusions. These results demonstrate that WL emission is ubiquitous in most flares, even down to C-class events.