Spectroscopic Case Studies of Four Long-duration Transition-region Explosive Events

Abstract

This work presents a detailed spectroscopic case study of four long-duration transition-region (TR) explosive events (EEs) observed in NOAA Active Region 13213 on 2023 February 10 using the Interface Region Imaging Spectrograph. The dynamic spectral evolution of each event is tracked through multicomponent Gaussian fitting of the Si IV 1403 Å line profiles. Three recurrent spectral morphologies are identified and characterized: bilateral wing enhancement, exclusive red-wing enhancement, and exclusive blue-wing enhancement, among which bilateral enhancement is the most common in the studied cases. Throughout their lifetimes of 20-25 minutes, these events display sustained and evolving bidirectional flows, with high-velocity components ($|v|$ > 100 km $s^{-1}$) emerging in late phases. These spectral signatures are interpreted as evidence of ongoing or recurrent magnetic reconnection, where bilateral profiles correspond to bidirectional outflows, and exclusive wing enhancements represent geometric or evolutionary phases of the same process. In contrast, cotemporal flare ribbons and loop structures exhibit pronounced, unidirectional redshifts. This study underscores that significant non-Gaussian wing enhancement, rather than exclusively high speed, constitutes a defining spectroscopic signature of EEs, and provides detailed kinematic constraints on the dynamics of such TR EEs.

Figures (7)

• Figure 1: Overview of transition region explosive events (EEs) in AR NOAA 13213, observed with IRIS slit-jaw images (SJIs) at 1400 Å and the corresponding Si4 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 Si4 1403 Å, while horizontal black dashed lines indicate the positions of EEs, flare ribbons, and loop structures.
• Figure 2: Panels (a) and (b): space–time maps of the total intensity of the Si4 1403 Å line and the corresponding intensity of explosive events (EEs), flare ribbons, and loop structures. Cyan contours delineate areas with an intensity 0.05 times the maximum intensity of observed region. Specific locations of EEs, flare ribbons, and loops selected for further analysis are marked by blue diamonds and black arrows. Panels (c) and (d): same as (a) and (b), but for the Doppler velocities.
• Figure 3: Panel (a): time evolution of the integrated intensity (black curve) and Doppler velocity (blue curve) of Si4 1403 Å line. The short vertical red lines indicate the specific position marked by blue diamonds in Figure \ref{['fig:map']}. Panel (b)--(k): the Si4 1403 Å line profiles of EE1. In each panel, the vertical dashed lines mark the rest wavelength of the Si4 1403 Å line. Green dashed curves indicate the composite Gaussian fits, while blue, red, and orange curves denote the individual Gaussian components. The global velocity, $v$, is derived from the centroid of the fitted profile. The velocities $v_{1}$, $v_{2}$, and $v_{3}$ correspond to the Doppler shifts of each Gaussian component.
• Figure 4: Same as Figure \ref{['fig:profile1']}, but for the line profiles of EE2.
• Figure 5: Same as Figure \ref{['fig:profile1']}, but for the line profiles of EE3.