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 perform a detailed multiwavelength analysis of the fine structures in the flare ribbon of a C3.9-class flare on 2021 April 22. A segment of the flare ribbon was rooted in a sunspot light bridge and exhibited discrete substructures that we term “burrs,” with equivalent diameters of 233–895 km and intercore separations of 1129–1739 km. These structures are characterized by discrete redshifted cores, accompanied by “tails” (length 700–1370 km and width 310–600 km) exhibiting faint blueshifts. These structures exhibit systematic slipping motions along the ribbon, with apparent velocities decelerating from about 40–21 km s −1 , and display a distinct quasiperiodicity of ∼6 minutes in H α and extreme-ultraviolet passbands. Differential emission measure (DEM) analysis confirms the emitting plasma is multithermal, dominated by temperatures of 1–2 MK. The observed morphology and kinematics are consistent with the scenario of impulsive energy deposition by precipitating plasmoids (oblique flux ropes) originating from tearing-mode fragmentation in the coronal current sheet. The specific spatiotemporal correlation between the tails and blueshifts supports the hypothesis of untwisting magnetic flux ropes. Furthermore, the ∼6 minute periodicity suggests that the reconnection process may be modulated by photospheric p -mode oscillations coupled with the tearing-mode instability. Our findings provide observational evidence that these light-bridge-anchored fine structures constitute elementary units of flare energy release.