Song Yongliang1,2 & Hui Tian2
1 Key Laboratory of Solar Activity, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China
2 School of Earth and Space Sciences, Peking University, Beijing 100871, China
Flares associated with a sudden enhancement of optical continuum emission are defined as white-light flares (WLFs). They are rarely observed, but important for flare research. It is widely believed that the magnetic field topology plays a significant role in solar eruptions. However, it is largely unknown whether it plays an important role in the production of WLFs. Recently, Hao et al. and Song et al. have identified WL emission in two circular-ribbon flares (CFs), which are believed to be associated with a fan-spine magnetic field configuration. Since flares sharing a similar shape of ribbon are usually associated with a similar magnetic field configuration, by investigating WL emission in CFs, we can examine whether the fan-spine magnetic field topology favors the production of WLFs.
Figure 1| AIA 1600 Å images, HMI line-of-sight magnetograms, and HMI continuum running difference images taken around the peak times of two CFs. The red dashed lines mark the outer edges of the circular ribbons in the 1600 Å images. The top shows a non-WLF, and the bottom shows a WLF.
From nearly eight-year observations of the SDO, we have identified 90 CFs from 36 ARs, including 8 X-class flares, 34 M-class flares, 47 C-class flares, and 1 B-class flare. Among these 90 CFs, 33 of them are WLFs, including 8 X-class CFs, 21 M-class CFs, and 4 C-class CFs. Thus, the occurrence rate of WLFs is about 37% (33/90) for CFs. The occurrence rate is even larger, about 69% (29/42), for CFs greater than M1.0. This is much higher than the previously-reported occurrence rates, suggesting that the fan-spine magnetic-field topology favors the occurrence of WLFs. However, only about 8% (4/48) of the C- and B-class CFs are WLFs. It is also worth noting that some ARs have each produced several CFs and nearly all of them are WLFs (see Figure 2).
Some parameters, including the area of flare region, flare duration, radial component of the photospheric magnetic field strength and electric current, and HXR power-law index, are derived and compared between WL CFs and non-WL CFs. Our analysis suggests that the CFs with WL enhancement tend to have a smaller spatial scale, shorter duration, and stronger and more complicated magnetic field.
Figure 3| Relationship between WL enhancement and the peak of GOES 1–8Å X-ray flux. The average (dIawl) and maximum (dImwl) values of WL enhancement are shown on the left and right, respectively. The green dashed line in each panel corresponds to the GOES class of X1.0.
We find that for X-class WL CFs, the WL enhancement has a positive correlation with the flare class (see Figure 3). This result, together with the fact that all the identified X-class CFs are WLFs, suggests that the magnitude of WL enhancement in large flares is largely determined by the amount of the released energy. However, for M- and C-class WL CFs, there is no such obvious correlation between the WL enhancement and flare class (see Figure 3). This suggests that other factors such as the time scale, spatial scale, and magnetic field complexity may play important roles in the generation of WL emission if the released energy is not high enough. For instance, if the released energy is low (C and B class), and the spatial and temporal scales are extremely small, the released energy may still produce rapid and efficient heating of the lower atmosphere, leading to detectable WL enhancement.
We noticed that in other magnetic field configurations even some X-class flares may not be WLFs. This observational fact, together with our findings about CFs, suggests that the energy as measured by the GOES classification is not a sufficient condition for the production of WL emission. The right magnetic field configuration may also be needed as well as enough energy.
For details of this work, please refer to our recent publication:
Song, Y. L & Tian, H. 2018, ApJ, 867, 159
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