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

TL;DR

This study uses high-resolution observations from SST and AIA to analyze the fine-scale structure of solar flare ribbons, revealing small, elongated features called riblets that support the patchy reconnection model.

Contribution

It introduces a detailed analysis of flare ribbon substructures using machine learning and spectroscopy, providing new insights into the fragmented nature of magnetic reconnection zones.

Findings

Riblets are 110-310 km wide and 620-1220 km long, with a maximum of 2000 km.

Riblets exhibit redshifted emission with velocities of 16-21 km/s.

Flare ribbons are composed of fragmented, vertically extended substructures.

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.