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

TL;DR

This work analyzes fine-scale structure in solar flare ribbons using co-aligned AIA and SST observations of three flares. It identifies riblets—elongated, jet-like features forming the ribbon fronts—via k-means clustering of H$\beta$ wing spectra and quantifies their emission with Gaussian fits, revealing widths of $110$-$310$ km and vertical extents of $620$-$1220$ km, along with redshifted velocities of $16$-$21$ km s$^{-1}$. The ribbons are not continuous but comprise vertically extended substructures, consistent with a fragmented reconnection region and patchy tearing in the coronal current sheet. These results support tearing-mode reconnection as a mechanism for ribbon-front formation and underscore the value of high-resolution SST observations for constraining flare dynamics; future DKIST/EST data and larger samples will improve statistics and physical interpretation.

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.

Figures (15)

• Figure 1: Context of the ARs that generated the flares. The columns from left to right show the M4.6, the C8.3, and the M1.8 flares, respectively. The rows from top to bottom show the AIA 171 Å, CaII 8542 Å core and H$\beta$ core channels, respectively. Each panel show the flare near the GOES peak time and the timestamps are shown in the lower-right corners. The GOES X-ray plot is added in the lower left corner in the upper row where the x-axis is in minutes relative to the image. The red and cyan contour highlights the FOV of CRISP and CHROMIS, respectively. The green and blue contours in the middle row show the CRISP magnetogram at $\pm$ 500 G. The red boxes in the lower row correspond to the FOVs in Figs. \ref{['fig:F1_kmeans_and_Gauss_maps']}--\ref{['fig:F3_kmeans_and_Gauss_maps']} respectively. An animation of this figure is available online.
• Figure 2: Fine-scale structures or riblets in a flare ribbon located near the western limb in a complex photospheric magnetic field configuration (flare F1). The right panel is a smaller FOV, as highlighted by the red rectangle in the left panel, revealing the fine-scale structures in a ribbon from a side-view. The colourmaps in the left and right panels are in logarithmic and linear scales, respectively.
• Figure 3: Riblets along the F1 ribbon. All panels show the same FOV, highlighting the central parts of the eastern ribbon. All panels except panel (c)--(d) show images in the H$\beta$ +0.8 Å channel. Panel (c) shows the wing-subtracted colourmap at H$\beta \pm$ 0.8 Å. Panel (a) shows the ribbon with identified riblets overplotted in green, red or cyan. Green pixels represent single-peaked RPs, red pixels represent double-peaked RPs with a stronger red peak, and cyan pixels represent near-symmetric double-peaked RPs. Similar is shown in panel (d) overplotted on the H$\beta$ core image. The resulting Doppler shifts and profile widths obtained from fitting the pixels are shown in panel (e)--(f), respectively. The black pixels in panel (e) mask the pixels where blue shifts were estimated. The contours of these pixels are added in panel (c). An animation related to this figure is available online.
• Figure 4: Same as Fig. \ref{['fig:F1_kmeans_and_Gauss_maps']} but for the F2 flare. The superimposed pixels in panel (f) are replaced with the estimated amplitude $A$ from profile fitting. An animation related to this figure is available online.
• Figure 5: Same as Fig. \ref{['fig:F1_kmeans_and_Gauss_maps']} but for the F3 flare. The overplotted colours in panel (f) are replaced with the estimated amplitude $A$ from profile fitting. An animation related to this figure is available online.