Statistics of transition-region loop brightenings and their heating implication

Transition-region loops are a type of critical magnetic structure in the solar atmosphere, yet their physical properties and evolutionary characteristics remain statistically poorly constrained. We aim to statistically characterize the physical properties of propagating brightening events in transition-region loops and to explore the underlying heating mechanism responsible for these brightenings. Using coordinated observations from the Extreme Ultraviolet Imager on board the Solar Orbiter and the Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory, we analyzed 42 propagating brightening events in loops that are unambiguously detected in both sets of instrument data. Each of these events evolves simultaneously in the AIA 94, 131, 171, 193, 211, 304, and 335 Å passband images, suggesting that they are in the transition-region or low-coronal temperature range. Our analyses show that these brightenings are impulsive, with an average brightening time of 118.4 ± 12.0 and a mean intensity decreasing time of 159.4 ± 16.6 . The propagating brightenings are predominantly subsonic, with velocities in the range of 0--90 ̨ms and an average of 51.3 ± 5.6 ̨ms. The lengths of brightenings range from 3 to 11 Mm, with an average and standard deviation of 6.3 ± 0.4 s s Mm , which are closely related to the propagation velocity and the lifetime. The initial brightening sites are predominantly located near the footpoints of these loops, and the number of brightening events decreases systematically with increasing loop height. Our results are consistent with an energizing mechanism regulated by enthalpy flows and radiative cooling. Dynamic magnetic evolution, including migration and cancellation, are taking place at the footpoints of these loops, and thus we suggest that magnetic cancellation, driven by reconnection and/or braiding that generate these brightenings, is a possible mechanism of plasma heating in a transition-region loop. Based on the measurements of these brightenings, we also propose a diagnostic method for the temperature or density of transition-region loops.