Quasiperiodic Pulsations in the Balmer Continuum in an X-class Solar White-light Flare

Nugget
Number:	499
1st Author:	De-Chao SONG
2nd Author:	et al.
Published:	June 30, 2025
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Previous Nugget:	High-Resolution Observations of a C3 class White-Light Flare

Introduction

Over 160 years ago, around 11:18 AM GMT on September 1, 1859, Richard Carrington and Richard Hodgson independently reported the first solar white-light flare (WLF), which was also the first solar flare that had been reported.

It is widely recognized that electromagnetic radiation enhancements during most solar flares predominantly occur in invisible bands, such as X-ray and ultraviolet, with only marginal enhancements in the white-light continuum. Compared to these "ordinary" flares, WLFs are particularly special due to their wider solar impact (potentially reaching down to the bottom of the photosphere), smaller enhanced emission areas, higher radiation power per unit area, and shorter durations. Thus, WLFs provide valuable examples for improving flare models. Our research (Ref. [1]) describes interesting pulsations seen, for the first time, in the energetically significant Balmer continuum. We think of this as something like the heartbeat of the flare, owing to its implied power.

Observations

On December 14, 2023, a powerful X2.8 class double-ribbon WLF (SOL2023-12-14T17:02) erupted. This event was observed with high cadence (1-2 s) in the Balmer continuum at 3600 Å by the White-light Solar Telescope (WST) onboard the ASO-S. The flare was simultaneously observed by multiple other instruments, such as the satellites SDO, SolO, Fermi, GOES, PROBA2, plus the ground-based EOVSA.

Results

For the first time, quasi-periodic pulsations (QPPs) with a fundamental period of approximately 20 s and a harmonic period of around 11 s were clearly detected in the Balmer continuum (see Figure 1) during the impulsive phase of the flare. This rare "heartbeat" QPP signature was corroborated by other instruments observing across different wavelengths.

Figure 1: Wavelet analysis for the Balmer continnum observations from WST. The top and bottom panels show the detrended light curve and wavelet power spectrum (WPS) observed in the flare region during the impulsive phase. Fundamental and harmonic QPP periods are indicated by the magenta and cyan arrows, respectively, and enclosed in blue rectangles

Further Fourier analysis indicated stronger harmonic QPP power from the eastern flare ribbon, notably contributing to both the fundamental (20 s) and harmonic (11 s) periods (e.g., the region within the blue circle in Figure 2). Interestingly, some regions on the western flare ribbon exhibited significant power at the 11 s period but minimal contribution at 20 s (e.g., the region within the green circle in Figure 2). Meanwhile, some other regions showed the opposite pattern (e.g., the region within the purple circle in Figure 2).

Figure 2: Spatial distribution of the QPP sources around the periods of 11 s and 20 s. Panel (a1) presents the temporal-averaged base difference map over the dominant time interval (i.e., TR1, also see Figure 3(g)) of the 11 s QPP, while panel (a2) displays the temporal-and-frequency-averaged Fourier power map for the same interval. Its dominant domain in time and frequency is enclosed by the smaller blue rectangles in the WPS diagrams in Figure 1. Panels (b1)(b2) have the same annotations as (a1)(a2) but correspond to the 20 s QPP.

According to multi-instrument observations, magnetic field extrapolations, and MHD theory, we suggest that kink-mode oscillations in flare loops may modulate the harmonic QPP generation. Combined with spatial distribution of the harmonic QPP sources (Figure 2), we propose that harmonic MHD oscillations (likely in the kink mode) in the majority of loops, and fundamental oscillations in a minority of loops may coexist.

Moreover, high-cadence imaging from WST revealed that the appearance of harmonic QPPs was highly synchronous with rapid flare ribbon elongation and separation motions (see Figure 3), suggesting that these "heartbeat" QPP signals are closely related to the basic energy conversion producing the flare.

Figure 3: Relationship between the harmonic QPPs and flare ribbon evolution. The magenta lines (AB and CD) in the upper right panel illustrate positions corresponding to time-distance diagrams (a) and (b), used to study rapid flare ribbon motions. Yellow lines in (a) and (b) indicate rapid ribbon separation and elongation movements, synchronized in time with harmonic QPP generation (cyan dashed lines). The bottom three rows show wavelet analysis results within blue, green, and purple regions.

Conclusions

Utilizing multi-band, high-cadence observations, this study investigates the emission properties of an X2.8 WLF, uncovering for the first time harmonic QPPs in the Balmer continuum at 3600 Å. This component of the emission spectrum of the flare contains a substantial fraction of the radiated energy, leading us to think of these oscillations as the heartbeat phenomenon. Future research will further investigate the underlying physical mechanisms using additional events and high-resolution observations.

Acknowledgements

The co-authors of the work described here and in Ref. [1] are Marie DOMINIQUE, Ivan ZIMOVETS, Qiao LI, Ying LI, Yang SU, B.A. NIZAMOV, Ya WANG, Andrea Fracesco BATTAGLIA, Jun TIAN, Li, FENG, Hui LI, and W.Q. GAN

References

[1] "Unveiling Spatiotemporal Properties of the Quasiperiodic Pulsations in the Balmer Continuum at 3600 Å in an X-class Solar White-light Flare"