OVSA Science Highlight No. 1: Microwave Precursor of a Major Solar Eruption

Contributed by Yuankun Kou^1,2,3 (¹ School of Astronomy and Space Science, Nanjing University, Nanjing 210023, China; ² Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, China; ³ School of Physics and Astronomy, University of Glasgow, Glasgow G12 8QQ, UK); Edited by Bin Chen; Posted on August 2, 2025.

Microwave sources overlaying on SDO/AIA 131 Å images during the precursor phase (panels (a) and (b)) and main phase (panels (c) and (d)) of the solar eruption on 2022 March 30.

Spatially resolved microwave brightness temperature spectra derived from the precursor phase (panels (a) and (b)) and main phase (panels (c) and (d)), respectively. The spectra during the precursor phase are consistent with thermal emission from heated plasma of a few million K.

Coronal mass ejections (CMEs) are the most energetic phenomena occurring in the solar atmosphere. To understand their initiation and early kinematic evolution, researchers have focused on progenitor features of eruptions, such as sigmoids, filaments, and hot channels, observed in extreme-ultraviolet (EUV) or X-ray bands (see Cheng et al. 2017 for a review). These progenitors are now recognized as distinct manifestations of the pre-eruptive magnetic field configuration, which undergoes energy accumulation for the later eruption.

In some recent works (Cheng et al. 2020, Cheng et al. 2023; Xing et al. 2024), a precursor phase of solar flares is emphasized as the period when CME progenitors transition from the quasi-static phase to the impulsive acceleration phase. During this precursor phase, the rise of CME progenitors and the energy release process are found to be slow and moderate, respectively, compared with the flare main phase. They also found from hard X-ray (HXR) spectra that the electron distribution in the precursor phase is thermal-dominated. However, microwave observations for the precursor phase are very rare, and all previous reports were based on spatially unresolved data (e.g., Altyntsev et al. 2012; Wang et al. 2017; Liu et al. 2022).

A new study led by Yuankun Kou and colleagues presents the first microwave imaging spectroscopy analysis of the precursor phase of a major solar eruption. Combining microwave data from the Expanded Owens Valley Solar Array (EOVSA) and EUV data from the Atmospheric Imaging Assembly (AIA) on NASA’s Solar Dynamics Observatory, the team examined a solar flare that erupted on March 30, 2022.

The event began with the appearance of a hot channel—a bright, elongated plasma structure above the magnetic polarity inversion line seen in EUV. Over a period of minutes, this structure slowly rose and expanded, emitting faint microwave signals (Figure 1). Intriguingly, these emissions came mostly from thermal electrons trapped in the hot channel and nearby precursor loops. Spatially resolved microwave spectra showed no significant non-thermal components at this stage (Figures 2a,b), indicating that the energy release was dominated by plasma heating rather than particle acceleration. By contrast, once entering the flare main phase, non-thermal tails of the electron energy distribution appeared and became even harder as the microwave emissions reached the peak flux; the plasma temperature also experienced a significant rise during this period (Figures 2c,d).

The study supports the scenario that the precursor phase, when the CME progenitor is present, is powered by a moderate form of magnetic reconnection, largely different from the fast reconnection during the flare main phase. The precursor reconnection might be in a hyperbolic flux tube (Titov et al. 2003) configuration, which drives the build-up, heating, and slow rise of the CME progenitor toward the explosive eruption.

---

Based on the recent paper by Kou, Y., Cheng, X., Yu, S., Luo, Y., Chen, B., & Ding, M. (2025), “Microwave Imaging Spectroscopy Diagnosis of the Slow-rise Precursor of a Major Solar Eruption,” The Astrophysical Journal Letters, doi:10.3847/2041-8213/adf063

References

• Cheng, X., Guo, Y., & Ding, M. (2017), Science China Earth Sciences, 60, 1383
• Cheng, X., Zhang, J., Kliem, B., et al. (2020), The Astrophysical Journal, 894, 85
• Cheng, X., Xing, C., Aulanier, G., et al. (2023), The Astrophysical Journal Letters, 954, L47
• Xing, C., Aulanier, G., Cheng, X., Xia, C., & Ding, M. (2024), The Astrophysical Journal, 966, 70
• Altyntsev, A. A., Fleishman, G. D., Lesovoi, S. V., & Meshalkina, N. S. (2012), The Astrophysical Journal, 758, 138
• Wang, H., Liu, C., Ahn, K., et al. (2017), Nature Astronomy, 1, 0085
• Liu, N., Jing, J., Xu, Y., & Wang, H. (2022), The Astrophysical Journal, 930, 154
• Titov, V., Galsgaard, K., & Neukirch T. (2003), The Astrophysical Journal, 582, 1172