High-Resolution Observations of a C3 class White-Light Flare

Nugget
Number:	498
1st Author:	Zhe XU
2nd Author:	Xiaoli YAN
Published:	June 16, 2025
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Previous Nugget:	The Sun's open-closed flux boundary and the origin of the slow solar wind

Introduction

White-light flares (WLFs) are intense brightenings in the visible continuum and signify substantial energy deposition in the lower solar atmosphere. While typically associated with powerful X- and M-class flares, this study (Ref. [1]) presents the compelling case of a WLF in a relatively weak flare at GOES class C9.3, SOL2023-09-11T06:01. This event produced strong white-light signatures and photospheric impacts and was observed across the spectrum as shown in Figure 1.

Figure 1: Panels (a)(c): AIA 171 Å, 1600 Å, and CHASE continuum images showing the general view of the flare SOL2023-09-11T06:01 in NOAA AR 13431. Panel (d): GOES SXR and ASO-S hard X-ray (HXR) fluxes of the white-light flare. Panel (e): ASO-S HXR background-subtracted count spectrum at the peak time of the emission.

This event, observed near the solar limb within NOAA AR 13431 on 11 September 2023, offered high-resolution insights thanks to coordinated observations by several space and ground-based observatories. Unlike typical electron beam-driven models, which often fall short of explaining photospheric effects, this event provides a new observational support for Alfvén wave pulses (Ref. [2,3]) as a viable energy transport mechanism during solar flares.

Multi-Instrument Observations

The WLF was captured simultaneously by several visible-light observers: the NVST TiO band at 7057 Å, the SDO/HMI continuum at 6173 Å, and the CHASE Fe I continuum level at 6569 Å. Two compact white-light kernels were observed aligned with the penumbral fibrils of a large sunspot, which further suggests photospheric involvement. Hard X-ray (HXR) data from ASO-S/HXI revealed that non-thermal electrons involved had energies below 50 keV and were insufficient alone to account for the observed white-light emission and photospheric magnetic field changes.

Alfvén Wave Signatures and Magnetic Field Amplification

The most remarkable aspect of this event is the detection of flare-induced vortex flows and a sudden magnetic-field amplification in the white-light kernel region (Figure 2). Optical flow analysis revealed a counterclockwise photospheric vortex, while vector magnetograms indicated a 400 G enhancement in magnetic field strength localized to the WLF core.

Figure 2: NVST TiO images (panels (a-d)) and SDO vector magnetograms (panels (g-h)) illustrating the sudden vortex flow and magnetic field amplification in the white-light flare region. Red arrows in panels (e) and (f) represent the photospheric flows before and during the flare. Figure 3 expands these two panels.

Figure 3: Panels (e) and (f) of Figure 2.

The energy required for this magnetic amplification (~4.1 x 10²⁸ erg) is comparable to the estimated nonthermal electron energy, suggesting a shared energy reservoir. Based on observed perturbations and Alfvén wave theory, we estimate that the energy carried by Alfvén waves is on the order of 10³⁰ erg. Only a small fraction reaches the photosphere, consistent with Alfvn wave dissipation in the chromosphere (Refs. [2,3]).

The observed spatial-temporal pattern of energy transfer, combined with the photospheric vortex responses, supports the idea that Alfvén waves can penetrate to the deep photosphere and contribute to white-light formation and magnetic field changes.

Conclusions

This study offers a clear observational link between Alfvén wave pulses and deep atmospheric responses during a solar flare. Even in a C-class event, the presence of white-light emission, photospheric vortex flows, and magnetic field enhancement point toward a multi-mechanism energy transport scenario, where both nonthermal electrons and Alfvén wave dissipation play critical roles

References

[1] "High-resolution Observations of a C9.3 White-light Flare and Its Impact on the Solar Photosphere"

[2] "Impulsive Phase Flare Energy Transport by Large-Scale Alfv{\'e}n Waves and the Electron Acceleration Problem"

[3] "Alfvénic Wave Heating of the Upper Chromosphere in Flares"