The M- and X-class White-light Flares in Super Active Region NOAA 13664/13697

Nugget
Number:	513
1st Author:	Zhichen JING
2nd Author:	and Ying LI
Published:	January 5, 2026
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Previous Nugget:	Iron Fluorescence in X-class Solar Flares

Introduction

White-light flares (WLFs) are a special kind of flare, in which an enhancement in the optical continuum appears. After the very first example was observed in 1859, only a small fraction of subsequent flares have exhibited this property. Can we associate such flares with particular properties of their parent active regions? Compared with ordinary active regions, the "super active regions (SARs)" are larger in area and more active in producing major flares. Are SARs more likely to produce WLFs? Do the WLFs from SARs have any special characteristics? These questions are interesting and remain to be explored. Two SARs, NOAA 13664 and 13697 (the same active region in two solar Carrington rotations), were visible on the solar disk during May and June 2024. They produced more than 100 M- and X-class flares in total, most of which were observed in the 360±2 nm waveband by the White-light Solar Telescope (WST) of the Lyα Solar Telescope (LST) on board the Advanced Space-based Solar Observatory (ASO-S).

Occurrence Rate of WLFs in SARs NOAA 13664 and 13697

We identified 48 WLFs at 360 nm from 89 M- and X-class flares in NOAA 13664/13697, with an occurrence rate of 54%. This occurrence rate is much higher than that (24%) in our long-term-continuous sample from October 2022 to May 2023 with WST data (Ref. [1]), in which 49 WLFs at 360 nm are identified from 205 M- and X-class flares. Note that the observing wavelength (360 nm) is a novel capability of the ASO-S observatory, and that most previous WLF observations have been at longer wavelengths. The main reason is that the two SARs have a relatively higher proportion of the X-class flares (19.1% vs. 3.4%), as the occurrence rate rises with flare magnitude. We further investigate the relationship between the WLF occurrence rate and sunspot number from October 2022 and June 2024, as shown in Figure 1. The WLF occurrence rate shows a similar fluctuating increasing trend as the sunspot number, with a Pearson correlation coefficient (Pcc) of 0.64 between them.

Figure 1: a) Time profile of the sunspot number (solid curve) and its smoothed profile (dotted curve) during the period of October 2022 and June 2024. (b) Temporal evolution of the WLF occurrence rate (magenta curve, corresponding to the right axis), together with the smoothed sunspot number (dotted curve) during the period. The red and blue asterisks in this panel (and also in panel (c)) mark the occurrence rates of WLFs in NOAA 13664 and 13697, respectively. (c) Scatter plot of the WLF occurrence rate vs. the smoothed sunspot number. The straight line is their linear fit.

The Physical Properties of the WLFs

We further analyze 22 on-disk WLFs with full WST data from NOAA 13664 and 13697. Compared with the long-term-continuous sample, these 22 WLFs from SARs have smaller maximum pixel enhancement (rp), maximum mean enhancement (rm), and brightening area (S), but longer duration (τ) (Figure 2). This may be related to the physical mechanisms of WLFs. We find that the proportion of WLFs with a similar peak time between the WL and hard X-ray (HXR) emissions in the two SARs (59.1%) is lower than that in the long-term-continuous sample (82.1%), which display the Neupert effect and have larger rm, rp, and S but shorter &\tau;. In addition, we find that most WL parameters (rp, S, τ) exhibit good positive correlations with the peak SXR flux and WL energy at 360 nm (Figure 3). In particular, the energy and duration have a better power-law relationship with an index of 0.35 (τ ~ E^0.35) (Figure 3(j)), which is very close to a theoretical value of 1/3 as well as similar to that for superflares on Sun-like stars (Ref. [2]).

Figure 2: Histograms of WLF parameters, including the maximum pixel enhancement (rp) (a), maximum mean enhancement (rm) (b), brightening area (S) corrected for projection effect (c), and duration (τ) (d). Results for the 22 WLFs from NOAA 13664 and 13697 are shown in red, and those for WLFs from the long-term-continuous sample (Ref. [1]) are in green for comparison.

Figure 3: Top panels show the relationships of maximum pixel enhancement (rp), maximum mean enhancement (rm), corrected area (S), WL duration (τ), and WL energy (E) with peak SXR flux for the 22 WLFs in NOAA 13664 and 13697. Bottom panels exhibit the relationships of E with rp, rm, S, and τ. When the correlation (Pcc) value (in red) is greater than 0.40, we make a linear fit (see the black curve).

Conclusions

The occurrence rate of WLFs at 360 nm in SARs NOAA 13664 and 13697 is as high as 54%, related to the unusually high proportion of X-class flares from these regions. Moreover, the WLF occurrence rate generally increases as the solar maximum approaches. Compared with our previous long-term-continuous sample, the WLFs in these two SARs show smaller relative enhancement and area and longer duration, which may be due to a lower proportion of WLFs with a similar peak time between the WL and HXR emissions. The WL parameters, including enhancement, area, and duration, increase with flare magnitude and WL energy. The WL duration and energy also follow a power-law relation (τ ~ E^0.35), similar to stellar flares from solar-type stars. Overall, the results suggest that SARs are more likely to produce WLFs than ordinary active regions, and the WLFs in SARs share some common characteristics with stellar flares.

References

[1] "A Statistical Study of Solar White-Light Flares Observed by the White-Light Solar Telescope of the Lyman-Alpha Solar Telescope on the Advanced Space-Based Solar Observatory (ASO-S/LST/WST) at 360 nm"

[2] "Statistical properties of superflares on solar-type stars based on 1-min cadence data"