A research team at the Yunnan Observatories (YNAO) of the Chinese Academy of Sciences has recently uncovered the fine spectral evolution of long-duration explosive events (EEs) through a spectroscopic case study of four EEs in the solar transition region.

Using high-resolution observations from the Interface Region Imaging Spectrograph (IRIS), the study identified three recurrent spectral morphologies. It highlighted that significant non-Gaussian wing enhancement, rather than high speed alone, was the defining spectroscopic signature of EEs. The finding provided new observational constraints for understanding small-scale explosive phenomena in the solar transition region.

Led by ZHOU Yi'an, a postdoctoral researcher at the YNAO, in collaboration with Prof. YAN Xiao, the study was recently published in The Astrophysical Journal.

The solar transition region serves as a critical boundary layer between the chromosphere and the corona. Small-scale EEs occurring there often exhibit non-Gaussian spectral line profiles and plasma motions reaching speeds of up to hundreds of kilometers per second, commonly interpreted as signatures of magnetic reconnection. However, the long-term spectral evolution of such events had remained poorly understood.

Focusing on four EEs (EE1–EE4) observed on February 10, 2023, each lasting 20 to 25 minutes, the team performed multi-component Gaussian fitting of the Si IV 1403 Å line profiles. This allowed them to track the spectral evolution throughout the entire lifetime of each event.

The results revealed three recurring spectral morphologies: bilateral wing enhancement, exclusive red-wing enhancement, and exclusive blue-wing enhancement. Among these, bilateral enhancement was the most common. Throughout the events, sustained and evolving bidirectional flows were observed, with high-velocity components (exceeding 100 km/s) emerging in the late phases. In contrast, the simultaneously observed flare ribbons and loop structures exhibited unidirectional redshifts.

The study suggested that bilateral enhancements are the direct spectroscopic signature of bidirectional reconnection outflows. Their persistence and the occasional appearance of high velocities in late phases suggested ongoing or renewed energy release.

This work was supported by the Strategic Priority Research Program of the Chinese Academy of Sciences, the National Natural Science Foundation of China, the Yunnan Key Laboratory of Solar Physics and Space Science, the Department of Science and Technology of Yunnan Province, and the Yunnan Revitalization Talent Support Program.

Overview of TR EEs in NOAA AR 13213, observed with IRIS SJIs at 1400 Å and the corresponding Si iv 1403 Å spectra near 00:33:33 UT, 01:17:50 UT, 02:44:39 UT, and 02:55:00 UT. In each SJI panel, the vertical white dashed line indicates the slit position. EE1–EE4 and the blue diamond denote the locations of EEs. Green contours outline regions with intensities exceeding 5% of the maximum within the field of view. In the spectral panels, vertical black dashed lines mark the line center of Si iv 1403 Å, while horizontal black dashed lines indicate the positions of EEs, flare ribbons, and loop structures. (Image by ZHOU Yi'an).