Fine details in solar flare ribbons: Statistical insights from observations with the Swedish 1-m Solar Telescope

Abstract

Flare ribbons serve as chromospheric footprints of energy deposition resulting from particle acceleration during magnetic reconnection. Their fine-scale structure provides a valuable tool for probing the dynamics of the flare reconnection process. Our goal is to investigate the fine-scale structure of flare ribbons through multiple observations of flares, utilising data obtained from the Atmospheric Imaging Assembly (AIA) and the Swedish 1-m Solar Telescope (SST). The aligned AIA and SST datasets for the three solar flares were used to examine their overall morphology. The SST datasets were specifically used to identify fine-scale structures within the flare ribbons. For spectroscopic analysis of these fine structures, we applied machine-learning methods (k-means clustering) and Gaussian fitting. Using k-means, we identified elongated features in the flare ribbons, termed as "riblets", which are short-lived and jet-like small-scale structures that extend as plasma columns from the flare ribbons. Riblets are more prominent near the solar limb and represent the ribbon front. Riblet widths are consistent across observations, ranging from 110-310 km (0".15-0".41), while vertical lengths span 620-1220 km (0".83-1".66), with a potential maximum of 2000 km (2".67), after accounting for projection effects. Detailed H-beta spectral analysis reveals that riblets exhibit a single, redshifted emission component, with velocities of 16-21 km s^1, independent of viewing angle. Our high-resolution observations of the three flare ribbons show that they are not continuous structures, but are composed of vertically extended, fine-scale substructures. These irregular features indicate that the reconnection region is not a smooth, laminar current sheet, but rather a fragmented zone filled with magnetic islands, consistent with the theory of patchy reconnection within the coronal current sheet.

arXiv comment14 pages, 15 figures, accepted for publication in A&A

References in corpus (26)

• The Interface Region Imaging Spectrograph (IRIS)
• The Solar Orbiter mission -- Science overview
• An Observational Overview of Solar Flares
• CRISP Spectropolarimetric Imaging of Penumbral Fine Structure
• CRISPRED: A data pipeline for the CRISP imaging spectropolarimeter
• Unprecedented Fine Structure of a Solar Flare Revealed by the 1.6~m New Solar Telescope
• Halpha line profile asymmetries and the chromospheric flare velocity field
• The European Solar Telescope
• SSTRED: Data- and metadata-processing pipeline for CHROMIS and CRISP
• On the 2012 October 23 circular ribbon flare: emission features and magnetic topology
• Evidence of Magnetic Helicity in Emerging Flux and Associated Flare
• Spectroscopic observations of evolving flare ribbon substructure suggesting origin in current sheet waves
• Fine structure of flare ribbons and evolution of electric currents
• Ultra-narrow Negative Flare Front Observed in Helium-10830~Å using the 1.6 m New Solar Telescope
• The transition from eruptive to confined flares in the same active region
• He I 10830 Å Dimming During Solar Flares, I. The Crucial Role of Non-Thermal Collisional Ionisations
• Development of a Confined Circular-cum-parallel Ribbon Flare and Associated Pre-flare Activity
• Correlated Spatiotemporal Evolution of Extreme-Ultraviolet Ribbons and Hard X-rays in a Solar Flare
• Is flare-ribbon fine structure related to tearing in the flare current sheet?
• Spectral variations within solar flare ribbons
• Generic low-atmosphere signatures of swirled-anemone jets
• $IRI{S}^{2+}$: A comprehensive database of stratified thermodynamic models in the low solar atmosphere
• Synthesizing Sun-as-a-star flare spectra from high-resolution solar observations
• Spectroscopic investigations of a filament reconnecting with coronal loops during a two-ribbon solar flare
• High Resolution Observations of the Low Atmospheric Response to Small Coronal Heating Events in an Active Region Core
• A High-resolution, Inversion-Based Synoptic Study of Solar Granulation

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