Multiwavelength Observations of Substructures in Solar Flare Ribbons

Solar flare ribbons are extensive brightenings in the chromosphere during flares, often showing fine-scale structuring that reflects the underlying energy release. Using high-cadence imaging from the Swedish 1 m Solar Telescope/CRISP during an X1.5-class limb flare on 2014 June 10, we identify and track 232 coherent, thread-like substructures, which we term for the first time “riblets.” From a statistical analysis, riblets have well-defined lifetimes and plane-of-sky speeds (typically 5–15 s and 50–150 km s −1 , respectively), establishing them as distinct ribbon substructures. From analysis of their temporal distributions, their distance–time ( X – T ) evolution uniquely reveals approximately linear and nonlinear (accelerating/decelerating) classes, a discrepancy that may be influenced by projection geometry. From analysis of their spatial distributions, we find no clear correspondence between the properties of adjacent riblets, suggesting that local atmospheric conditions (fine-scale thermodynamic and/or magnetic structuring) govern their kinematics more than spatial variations in electron beam energy flux. From analysis of their spectral distributions, clusters of riblets do show temporal and spatial coincidence with hard X-ray emission signatures, consistent with episodic electron beam injection into the chromosphere. Using Fermi/Gamma-ray Burst Monitor spectroscopy, we derive thick-target parameters suitable for flare simulations, with representative values δ low ≈ 5.93, E c ≈ 24.7 keV, and an implied beam energy flux F beam ≈ 1.5 × 1 0 10 erg cm −2 s −1 (based on RHESSI footpoint area). Together, these results identify riblets as the fundamental building block of flare ribbons and provide quantitative constraints for forward tests of riblet formation mechanisms.