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

(Solar Orbiter Nugget #86 by Ryan J. French¹, Maria D. Kazachenko^1,2,3, David Berghmans⁴, Elke D’Huys⁴, Marie Dominique⁴, Ritesh Patel⁵, Dana-Camelia Talpeanu⁴, Cole A. Tamburri^1,2,3, and Rahul Yadav^1,2)

Introduction

During solar flares, magnetic reconnection accelerates high energy particles from the flare current sheet, down magnetic loops, toward the Sun’s surface. As these high-energy particles reach the chromosphere, they deposit energy at the magnetic footpoints, heating local plasma to form flare ribbons. Flare ribbons, therefore, mark the magnetic connectivity between the chromosphere and flaring current sheet in the solar corona. Due to this magnetic connectivity, flare ribbon dynamics must in some way reflect the fundamental energy release processes within the flare current sheet [1], a region challenging to probe directly. Observations of flare ribbons can therefore offer insights into the physics dictating flare onset and evolution.

Flare ribbons are not smooth and laminar but exhibit internal variations within them. Studies from the Interface Region Imaging Spectrograph (IRIS) have found blob-like, hook-like, swirling and fragmented structures as small as ≈150–300 km in the flare ribbons [2, 3] Ribbon structures of similar sizes have been found in ground-based H-alpha images [4].

In this work, we present observations of bead-like ribbon kernel structures by the Solar Orbiter Extreme Ultraviolet Imager (EUI). We track intensity variations along the flare ribbon to reveal a multitude of bead behaviours. We compare these behaviours with expectations from current sheet processes in solar flares, specifically the tearing-mode instability [5].
 

Results

Figure 1. Overview of the 2024 March 24 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).

We present observations of a C9.9-class flare on 2024 March 24 from AR 13615 (Figure 1). The EUI High Resolution Imager in the EUV (HRIEUV) observed the flare with a pixel scale of 0.492”, corresponding to 140 km at Solar Orbiter’s heliocentric distance of 0.38 au. During this event, HRIEUV observed with a sequence of one standard-exposure (2 s) image, followed by six short-exposure (0.04 s) images, at a cadence of 2 s per image.

Figure 2 shows the evolution of the flare ribbon within the red field of view (FOV) outlined in Figure 1. Each row of images contains a sequence of pairs of near-simultaneous images (separated by 2 s) with standard exposure (top) and short exposure (bottom). The ribbon image sequence spans a subsection of the impulsive phase of the flare. Small, bright bead-like structures are visible in the flare ribbons throughout the flare. We implement flare ribbon tracking to track the central ribbon axis of the flare ribbon as the ribbon bright points evolve in time and space. By combining the intensity cross section measured for every time step, we create the time–distance stack plots in Figure 3.
 

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 2 s).

The stack plot in Figure 3 reveals a mixture of multiple bright points moving in different patterns and speeds. We highlight four behaviours in particular, labelled subregions 1–4 in Figure 3. The distinct subregion behaviours are the following:

1. Sub-region 1: Beads exhibit quasiperiodic “on-off” intensity pulsations as they travel along the flare ribbon. The pulsations continue beyond the subregion FOV.
2. Sub-region 2: Beads exhibit back-and-forth “zigzag” motion as they flow along the flare ribbon: This manifests itself as a “W” shape within the stack plot, extending beyond the subregion FOV.
3. Sub-region 3: Rapid bead motions along the ribbon, at speeds of ≈600 km s−1. Two beads coalesce to form a brighter one.
4. Sub-region 4: Stationary bright points at a fixed position in the ribbon, persisting for 4+ minutes.

In this nugget, we highlight the ribbon behaviour observed in sub-regions 1 and 2.

Figure 3. 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 lines in the middle panel mark the earliest start times and end time of exponential growth, as presented in Figure 5. The bottom panels present cropped subregions 1–4 from the middle panel, highlighting (1) QPP-like brightenings, (2) “zigzag” motions, (3) rapid motions, and (4) stationary bright points.

Discussion

Sub-region 1 shows quasiperiodic brightenings of the bead structures as they propagate along the ribbon, with periods around ∼15–30 s. There are several candidate processes potentially responsible for generating Quasiperiodic pulsations in flares, with previous observations of ribbon oscillations [6, 7] attributed to waves or turbulence in the coronal current sheet, consistent with that expected of the tearing-mode instability or bursty magnetic reconnection.

Sub-region 2 displays a different kind of oscillatory behaviour; an oscillation in the spatial position of the bright point. While flowing along the ribbon, the bead exhibits quasiperiodic “back-and-forth” motions, briefly changing direction before continuing in its original direction. This manifests itself as a “W” profile in Figure 3. This behaviour is unusual and to our best knowledge, has not been reported before. The origin of this behaviour is challenging to describe qualitatively, but it is potentially related to fine-scale hook-like structures found in flare ribbon simulations [8]. As they propagate with time, the hook-like structures curl up like a breaking ocean wave, before returning to a simpler structure. It is possible that these hook structures manifest as bead structures in observations. In simulations, this hook-like behaviour is attributed to the tearing-mode instability within the flare current sheet.

Interestingly, when we apply a Fast-Fourier Transform along the intensity cross-sections at each time step, we find evidence the ribbon scales forming at a key separation of 1.7–1.9 Mm before developing into more complex structures at progressively larger and smaller spatial scales. This behaviour is another key prediction of the tearing mode instability [9], where the current sheet tears at a fundamental spatial scale, before cascading (and inversely cascading) to energy release on larger and smaller scales. We believe we are seeing the imprint of that process in the flare ribbons.

Conclusions

The fast-cadence and non-saturated short-exposure EUI/HRIEUV observations reveal a mixture of different behaviours along the flare ribbon. Many of these ribbon behaviours have been observed independently before but never before together all at once. Although we do not believe a single process can account for all the ribbon behaviours, many observed ribbon behaviours are consistent with the presence of the tearing-mode instability. 

Affiliations

(1)    Laboratory for Atmospheric and Space Physics, University of Colorado Boulder, Boulder, CO 80303, USA
(2)    National Solar Observatory, 3665 Discovery Drive, Boulder, CO 80303, USA 
(3)    Dept. of Astrophysical and Planetary Sciences, University of Colorado, Boulder, 2000 Colorado Ave, Boulder, CO 80305, USA
(4)    Solar-Terrestrial Centre of Excellence—SIDC, Royal Observatory of Belgium, 3 Avenue Circulaire, B-1180 Uccle, Belgium
(5)    Southwest Research Institute, 1301 Walnut Street, Suite 400, Boulder, CO 80302, USA

Acknowledgements

Solar Orbiter is a space mission of international collaboration between ESA and NASA, operated by ESA. The EUI instrument was built by CSL, IAS, MPS, MSSL/UCL, PMOD/WRC, ROB, LCF/IO with funding from the Belgian Federal Science Policy Office (BELSPO/PRODEX PEA 4000112292 and 4000134088); the Centre National d’Etudes Spatiales (CNES); the UK Space Agency (UKSA); the Bundesministerium für Wirtschaft und Energie (BMWi) through the Deutsches Zentrum für Luft- und Raumfahrt (DLR); and the Swiss Space Office (SSO).

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