Evolution of Flare Ribbon Bead-like Structures in a Solar Flare

TL;DR

This study uses high-resolution, fast-cadence EUV observations from Solar Orbiter to analyze small-scale bead-like structures in solar flare ribbons, revealing dynamic behaviors and evidence for tearing mode instability.

Contribution

First high-resolution observation of bead-like flare ribbon structures at sub-1000 km scales, linking their formation to tearing mode instability in solar flares.

Findings

Bead structures form with 1.7-1.9 Mm separation.

Structures exhibit multiple dynamic motions including QPP-like brightenings.

Evidence supports tearing mode instability as a formation mechanism.

Abstract

We present fast cadence and high resolution observations of flare ribbons from the Solar Orbiter Extreme Ultraviolet Imager (EUI). Utilizing the short-exposure observations from the EUI High Resolution Imager in EUV (HRIEUV), we find small-scale blob/bead-like kernel structures propagating within a hook at the end of a flare ribbon, during the impulsive phase of a C9.9-class solar flare. These bead structures are dynamic, with well-resolved spatial separations as low as ~420-840 kilometers (3-6 pixels) - below the observable limit of full-disk solar imagers. We analyze the evolution of the plane-of-sky apparent velocity and separation of the flare ribbon structures, finding evidence for multiple processes occurring simultaneously within the flare ribbon. These processes include - quasi-periodic pulsation (QPP)-like brightenings, slow back-and-forth zig-zag motions along the ribbon, rapid apparent motions along the ribbon (600+ km/s), and stationary blob-like structures. Finally, we conduct Fast Fourier Transform analysis and analyze the start times of exponential growth in the power spectrum at different spatial scales across the flare ribbon. Our analysis reveals that the ribbon beads form with a key spatial separation of 1.7-1.9 Mm, before developing into more complex structures at progressively larger and smaller spatial scales. This observation is consistent with predictions of the tearing mode instability.