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

TL;DR

This study uses fast-cadence, short-exposure 174 Å imaging from Solar Orbiter/HRIEUV to resolve bead-like kernel structures within a flare ribbon during a C9.9-class event. Through ribbon tracking, sub-structure analysis, and spatial Fourier analysis, the authors identify bead separations of approximately $420-840$ km and a spectrum of behaviors, including quasi-periodic brightenings, zig-zag motions, rapid along-ribbon flows near $600$ km s$^{-1}$, and stationary points, with dominant spatial scales at $1.25$ and $1.67$ Mm and initial growth near $1.7-1.9$ Mm at rates $0.01-0.06$ s$^{-1}$. The results provide quantitative evidence for tearing-mode instability in the coronal current sheet, showing a cascade and inverse-cascade of spatial scales and highlighting the multi-process nature of flare ribbon dynamics. Overall, the work demonstrates the power of unsaturated, high-cadence EUV imaging to probe multi-scale reconnection processes in solar flares and strengthens the link between ribbon fine-structure and current-sheet instabilities.

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.

Figures (5)

• Figure 1: Overview of the 2024 March 24th C9.9-class solar flare. A) Full Sun EUI/FSI 174 Å image, cropped on the inner corona. The cyan FOV marks the EUI/HRIEUV FOV. B) Full (standard exposure) EUI/HRIEUV image, capturing the solar flare within active region AR 13615 (marked by the green box). C) Cropped (standard exposure) EUI/HRIEUV image, within the green FOV marked in panel B. D) Cropped short-exposure EUI/HRIEUV image, capturing the flare ribbons within the red FOV in panels B and C. E) X-ray and EUV time series of the solar flare, including STIX 6-12 and 12-25 keV emission, and light curves of (standard exposure) EUI/HRIEUV 174 Å emission within the FOV of panels C (standard-exposure) and D (short exposure). Panel E lightcurves are normalized between minimum and maximum values.
• Figure 2: EUI/HRIEUV 174 Å snapshots of solar flare ribbon evolution. Adjacent top/bottom panels show EUI/HRIEUV standard (2 s) and short (0.04 s) exposure images respectively, within the same FOV (the short exposure images precede the standard exposure images by two seconds). Snapshots span from 00:48:10 to 00:54:34 UT. The movie version of the figure shows the same images at full cadence, over the same time range.
• Figure 3: Demonstration of flare ribbon tracking. A) Sample short-exposure EUI/HRIEUV image, with the red line marking the tracking central ribbon axis along the flare ribbon. B) Intensity cross-section along the central ribbon axis shown in panel A. C) Spatial Fast Fourier Transform (FFT) of the intensity cross-section presented in panel B.
• Figure 4: Time-distance plot of intensity variations along the central ribbon axis, from standard exposure (top panel) and short exposure (middle panel) HRIEUV images. The dotted red vertical line in the middle panel marks the earliest start times and end time of exponential growth, as presented in Figure \ref{['fig:FFT']}. The bottom panels present cropped sub-regions 1-4 from the middle panel, highlighting 1) QPP-like brightenings, 2) 'zig-zag' motions, 3) rapid motions, and 4) stationary bright points.
• Figure 5: Spatial FFT Analysis. A) Time-FFT plot, showing FFT power along the flare ribbon at each time step. The red line marks the start time of exponential growth at each spatial scale. The horizontal gray dashed line shows the spatial scale power cross-section presented in panel B. The vertical green line marks the time of the example ribbon image used to demonstrate the tracking and FFT procedure in Figure \ref{['fig:tracking']}. B) Cross-section of the panel A Time-FFT plot, at a spatial scale of 1.7 Mm. The green line shows the smoothed growth curve, and solid red line the best exponential fit to the smoothed data (with a growth rate of $\approx$ 0.01 s$^{-1}$). The vertical dashed red line marks the time exponential growth starts at this spatial scale. C) The relative timing of exponential growth at different spatial scales (red curve, left axis), relative to the onset of HXR emission (blue/black curves, right axis). The vertical dashed gray line marks the end time of panels A and B.