Quasiperiodic Slipping Motion of Flare Ribbon Fine Structures Anchored in a Sunspot Light Bridge

Abstract

We used high-resolution observations from the New Vacuum Solar Telescope and the Solar Dynamics Observatory to carry out a detailed multiwavelength analysis of the fine structures in the flare ribbon of a C3.9-class flare on 22 April 2021. A segment of the flare ribbon was rooted in a sunspot light bridge and exhibited discrete substructures, which we term "burrs", with equivalent diameters of 233-895 km and inter-core separations of 1129-1739 km. These structures are characterized by discrete redshifted cores accompanied by "tails" with lengths of 700-1370 km and widths of 310-600 km that show faint blueshifts. The burrs display systematic slipping motions along the ribbon, with apparent velocities decreasing from about 40 to 21 km/s, and show a distinct quasi-periodicity of about 6 minutes in H-alpha and EUV passbands. Differential emission measure analysis indicates that the emitting plasma is multi-thermal and dominated by temperatures of 1-2 MK. The observed morphology and kinematics are consistent with impulsive energy deposition by precipitating plasmoids, or oblique flux ropes, produced by tearing-mode fragmentation in the coronal current sheet. The close spatiotemporal association between the tails and blueshifts supports the interpretation that these features are related to untwisting magnetic flux ropes. The approximately 6-minute periodicity further suggests that the reconnection process may be modulated by photospheric p-mode oscillations coupled with tearing-mode instability. These results provide observational evidence that light-bridge-anchored fine structures can act as elementary units of flare energy release.

Figures (6)

• Figure 1: Multiwavelength overview of the C3.9-class flare in AR 12816 on 2021 April 22. Panel (a): SDO/HMI LOS magnetogram showing the magnetic field distribution of the active region. The red and blue curves delineate the J-shaped positive-polarity ribbon and the L-shaped negative-polarity ribbon , respectively. Panel (b): SDO/AIA 171 Å image showing the brightened coronal loop structures during the flare. Panel (c): SDO/AIA 304 Å image clearly outlining the flare ribbons. Panel (d): NVST TiO image showing the fine optical structure of the main sunspot at the footpoint of the SR. Panel (e): NVST H$\alpha$ image displaying the corresponding chromospheric structure of the flare ribbons. Panel (f): SDO/AIA 131 Å image showing the post-flare loops formed in the later phase of the flare.
• Figure 2: Dynamic evolution of the flare ribbon footpoint in the sunspot. This sequence of co-aligned AIA 171 Å images and HMI continuum intensity images displays the continuous evolution of the flare ribbon footpoint near the sunspot from 04:14:10 UT to 04:40:58 UT. The images reveal the process from an initial curved bright arc (panels (a) and (b)) to several discrete bright kernels (panels (c)–(e)), which finally stabilize at the location of a light bridge (panel (f)).
• Figure 3: Multiwavelength observations of the structures of burrs along the flare ribbon. Panels (a)–(d): SDO/AIA 131 Å image sequence showing the appearance and motion of burrs in the hot corona. Panels (e)–(h): NVST H$\alpha$ image sequence showing the corresponding burr structures in the chromosphere; the red box in panel (f) indicates the FOV of panels (m)–(p). Panels (i)–(l): H$\alpha$ Doppler proxy map sequence revealing the Doppler proxy signals at the locations of these structures. Panels (m)–(p): NVST H$\alpha$ +0.4 Å off-band images of the burrs, where the black contours represent the boundaries defined by the FWHM method. The red, blue, and black arrows point to different burrs appearing at different times to track their movement; note that arrow colors are for feature tracking only and do not represent the sign of the Doppler velocity. The animation of this figure includes AIA 131 Å, NVST H$\alpha$, and Doppler proxy map images from 04:37 to 05:25 UT with a video duration of 4 s.
• Figure 4: Kinematic analysis of the slipping motion of the burrs using time-distance diagrams. Panels (a) and (b): Time-distance diagrams made from the NVST H$\alpha$ and AIA 131 Å data, respectively, along the slit "A–B" marked in Figure \ref{['fig:3']}(e).The slanted bright stripes in these diagrams illustrate the slipping motion of the burrs, and the red dashed lines indicate the measurements of the apparent velocity. Panel (c) is a time-distance diagram made from H$\alpha$ data along the slit "C–D" in Figure \ref{['fig:3']}(e), showing the intermittent eruptions of the burrs as indicated by the red arrows. Panel (d): The corresponding time-distance diagram of the Doppler map, displaying the Doppler proxy signals distribution at the locations of the bright stripes.
• Figure 5: Wavelet analysis of the H$\alpha$, AIA 131 Å, and AIA 171 Å light curves. Panel (a): The H$\alpha$ light curve extracted from the horizontal slice "L1" in Figure 4(a). Panel (b): The wavelet power spectrum of the light curve in panel (a) from 04:37 to 05:10 UT. The black contour outlines the 95% significance level. Panel (c): The corresponding global wavelet spectrum, showing a dominant period of $\sim$6 minutes, which is above the 95% confidence level (dashed gray line). Panels (d)–(f): Similar to panels (a)–(c), but for the AIA 131 Å light curve from slice "L1" in Figure 4(b). Panels (g)–(i): Similar to panels (a)–(c), but for the AIA 171 Å light curve extracted from the same slice "L1", which also reveals a consistent dominant period of $\sim$6 minutes.