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

TL;DR

This paper provides a detailed spectroscopic analysis of four long-duration transition-region explosive events, revealing recurrent spectral morphologies, sustained bidirectional flows, and signatures of magnetic reconnection.

Contribution

It introduces a comprehensive spectroscopic characterization of TR explosive events, highlighting the importance of non-Gaussian wing enhancements as key signatures.

Findings

Bilateral wing enhancement is the most common morphology.

Events show sustained bidirectional flows with velocities over 100 km/s.

Non-Gaussian wing enhancement is a key spectroscopic signature of EEs.

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 \r{A} 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.