Sun-as-a-star Analysis of a Solar Eruption Source Region Using H-alpha Spectroscopic Observations from CHASE

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Number:	510
1st Author:	Xiaofeng LIU
2nd Author:	Yijun HOU
Published:	November 24, 2025
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Previous Nugget:	On the Origin of Solar Long-Duration Gamma-Ray Flares

Introduction

Solar flares are among the most powerful events on the Sun. They are often accompanied by filament/prominence eruptions and coronal mass ejections (CMEs), which can interact with planetary magnetospheres and impact our daily life on Earth. Similarly, stellar flares and eruptive events on other host stars can also influence their planets. However, due to the limited stellar observation, the detailed physical mechanisms of stellar eruptive events have not yet been clearly clarified. Studying the Sun as if it were a star clearly makes the observational links exact. Therefore, spatially integral solar observed data have been widely used to compare with and interpret stellar observations: Sun-as-a-star analysis (e.g., Refs. [1] and [2]) can serves as a bridge for comparative studies of solar and stellar phenomena. It is well known that multiple physical processes occur simultaneously in the source region during a solar eruption, collectively shaping the overall Sun-as-a-star spectral characteristics (Ref. [3]). To better distinguish the spectral features of different physical processes in the solar eruption source region, we can dive into the subregion responsible for the phenomena.

Observations

We have selected (Ref. 4) five solar eruptive events dominated by different physical processes. These include the well-known X9 event SOL2024-10-03. These events all have relatively complete full-disk spectroscopic observations in the novel Hα (6563 Å) and Fe I (6569 Å) wave bands by the [Hα Imaging Spectrograph (HIS)] onboard the Chinese Hα Solar Explorer (CHASE). They were also observed simultaneously by multiple other observatories; notably SDO/AIA, SDO/EVE, GOES, and LASCO.

Results

By conducting Sun-as-a-star analyses of subregions within the source region, we elucidate typical temporal spectral characteristics of different dynamical processes (as shown in Figure 1). The Sun-as-a-star Hα line profiles in flare ribbon regions exhibit line center emission, line broadening, and red asymmetry, which are associated with chromospheric condensation. In the regions of the erupting filaments, the Hα spectra display a sequence of changes, from accelerating blueshifted absorption to decelerating blueshifted absorption, and finally to redshifted absorption, fully capturing the dynamic processes of the cold plasma within the filaments, including accelerated uplift, deceleration, and subsequent downward motion (the latter two processes typically occurring in partial and failed eruptions).

Figure 1: CHASE imaging and spectral observations of a flare event associated with a failed filament eruption. (a1)-(a3) Sequence of CHASE Hα blue wing, line center and red wing images. (b1)-(b2) Doppler velocity fields of the source region, derived from CHASE Hα spectra. (c1)-(c4) The time series of Sun-as-a-star Hα dynamic spectra of the entire source region, the filament lifting area, flare ribbons, and the filament falling area, respectively.

During intense solar flares, when the radiative enhancement from hot plasma in flare ribbons significantly outweighs the absorption effects caused by cold filament plasma, the spectral signatures of filament eruptions can become entirely obscured (as shown in Figure 2). This may partially explain the relatively small number of confirmed stellar CMEs in the Hα waveband - such events are often accompanied by superflares.

Figure 2: Hα line center imaging observation of SOL2024-10-03 (a), the CME captured by LASCO/C2 (b), and the corresponding Sun-as-a-star Hα dynamic spectra (c).

To further investigate the dynamics of plasma with different temperature in source region, we also analyzed ultraviolet spectral data from the SDO/EVE instrument, a Sun-as-a-star EUV spectrometer. By applying double-Gaussian fitting to the O V line profiles, we derived the velocity evolution of high-temperature plasma. The results reveal that the blue-shift velocities observed in the SDO/EVE O V line are generally higher than those derived from Hα spectra (as shown in Figure 3). These results indicate that the combination and comparison of Hα and UV spectral observations provide a more comprehensive characterization of the filament eruption process and offers improved observational constraints for identifying subsequent CMEs, which can also be used in stellar observations.

Figure 3: Composite dynamic spectra of Hα and O V during events associated with successful (a, b) and failed (c) filament eruptions. The color map represents the Sun-as-a-star integrated Hα spectra from the eruption source region, while the gray areas display the full-disk integrated O V spectra.

Conclusions

In this work (Ref. [4]), we have described Sun-as-a-star analysis on five solar eruptive events with distinct characteristics. Although the five events differ from one another, the spatially integral spectra of subregions, dominated by the same physical process, exhibit similar temporal spectral characteristics. We also find that the presence of blueshifted absorption features in the Hα waveband does not necessarily indicate the occurrence of a CME. Furthermore, the spectral signatures of filament eruptions may be obscured when the magnitudes of emission and absorption are markedly different. Finally, the combination and comparisons of Hα and UV spectral observations can effectively reveal the velocity evolution of erupting filaments and the potential existence of associated CMEs, depicting a more comprehensive scenario of the solar and stellar filament eruption.

Acknowledgements

This work (and Ref. [4]) is the result of a collaborative effort between the authors and Ying LI, Ye QIU, Ting LI, Yingjie CAI, Shihao RAO, Junyi ZHANG, and Chuan LI.

References

[1] "Probable detection of an eruptive filament from a superflare on a solar-type star."

[2] "Sun-as-a-star Analyses of Various Solar Active Events Using Hα Spectral Images Taken by SMART/SDDI"

[3] "Sun-as-a-star Analysis of the X1.6 Flare on 2023 August 5: Dynamics of Postflare Loops in Spatially Integrated Observational Data"

[4] "Sun-as-a-star Analysis of the Solar Eruption Source Region Using Hα Spectroscopic Observations of CHASE"