A recent study published in The Astrophysical Journal Letters by Dr. XU Zhe, an associate researcher at the Yunnan Observatories of Chinese Academy of Sciences, and his collaborators has presented high-resolution observations of a C9.3-class white-light solar flare, shedding new light on its impact on the solar photosphere.
This research not only challenges conventional understanding of white-light flare mechanisms but also provides rare observational evidence supporting Alfvén waves as a key energy transport channel in the solar atmosphere.
On September 11, 2023, a white-light flare occurred in National Oceanic and Atmospheric Administration (NOAA) Active Region 13431, located near the eastern limb of the Sun. Despite being classified as a relative moderate C9.3 flare which is typically considered insufficient to generate white-light emission, it exhibited clear white-light enhancements. Using advanced solar observatories, including the New Vacuum Solar Telescope (NVST), the ASO-S satellite, and the CHASE satellite, the researchers captured the detailed evolution of this unusual flare.
Figure 1: General view of the flare SOL2023-09-11T06:01 in AR 13431. Image by XU.
High-resolution TiO-band observations from NVST revealed two bright white-light kernels connected by filamentary structures aligned with the penumbral filaments of a nearby sunspot, indicating a substantial contribution from deeper layers of the photosphere.
Remarkably, sudden vortex motions and rapid magnetic field amplification were observed at the white-light flare kernel. These phenomena suggest that the released energy was rapidly transported from the corona to the photosphere via Alfvén wave pulses.
These transient photospheric vortex flows represent the arrival of Alfvén wave pulses propagating through the solar atmosphere. They not only strengthened the local magnetic field but may also have assisted non-thermal electrons in penetrating into deeper atmospheric layers, leading to the observed white-light emission.
Figure 2: NVST TiO images illustrating the sudden vortex flow and magnetic field amplification in the white-light flare region. Image by XU.
Hard X-ray imaging spectroscopy from ASO-S revealed non-thermal electrons with energies below 50 keV—energies insufficient to individually heat the lower photosphere. This finding supports a hybrid mechanism where Alfvén waves and electron beams work together to drive flare energy deposition. Energy estimations suggest that Alfvén waves carried as much as 10^30 ergs, which is enough to enhance local magnetic fields and accelerate particles.
Furthermore, a layer-dependent time delay was identified across different wavelengths observed in AIA 304 Å, Hα, and TiO bands. This layered timing is consistent with theoretical models of Alfvén wave propagation from the corona to the lower atmosphere.
This study demonstrates that even C-class solar flares, under specific magnetic and atmospheric conditions, can generate detectable white-light radiation. The results significantly broaden our understanding of white-light flare formation thresholds and contribute vital observational evidence for refining models of vertical energy transport in solar flares. The research also highlights the growing capabilities of China’s domestic solar observations in conducting high-cadence, multi-wavelength, and high-resolution studies of the Sun.
This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, the National Science Foundation of China (NSFC), the Yunnan Key Laboratory of Solar Physics and Space Science, and the Yunnan Province XingDian Talent Support Program.
Contact:
XU Zhe
Yunnan Observatories, CAS
E-mail: xuzhe6249@ynao.ac.cn
