Fine details in solar flare ribbons

Statistical insights from observations with the Swedish 1-m Solar Telescope

¹ Rosseland Centre for Solar Physics, University of Oslo PO Box 1029 Blindern 0315 Oslo, Norway
² Institute of Theoretical Astrophysics, University of Oslo PO Box 1029 Blindern 0315 Oslo, Norway
³ NASA Goddard Space Flight Center, Heliophysics Science Division Code 671 Greenbelt MD 20771, USA
⁴ Department of Physics and Astronomy, George Mason University Fairfax VA 22030, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 4 August 2025
Accepted: 27 October 2025

Abstract

Context. 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.

Aims. 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).

Methods. 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-scale structures, we applied machine-learning methods (k-means clustering) and Gaussian fitting.

Results. 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 Hβ spectral analysis reveals that riblets exhibit a single, redshifted emission component, with velocities of 16−21 km s⁻¹, independent of the viewing angle.

Conclusions. Our high-resolution observations of the three flare ribbons show that they are not continuous structures, but rather 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 (blobs or riblets), consistent with the theory of patchy reconnection within the coronal current sheet.