The role of the magnetic shear in reconnection-driven flare energy release

We present novel measurements of the shear of post-reconnection flare loops (PRFLs), and study its evolution with respect to the progress of magnetic reconnection and flare emission. The flare SOL20141218T21:40 was observed by the Solar Dynamics Observatory’s Atmosphere Imaging Assembly and the Ramaty High Energy Solar Spectroscopic Imager. It exhibits two quasi-parallel ribbons adjacent to the magnetic polarity inversion line (PIL), spreading in time first parallel to the PIL and then mostly in a perpendicular direction. Anchored at the flare ribbons, a large number of PRFLs form and emit in extreme ultraviolet passbands. We measure the shear of the PRFLs as the complement of the angle of these loops with the PIL.The median shear decreases from 60-70 degrees to less than 20 degrees, on a time scale of ten minutes, during which flare emission, particularly the >30 keV hard X-ray emission, rises toward the peak. We find that, in this flare, the shear-modulated total reconnection rate better matches the non-thermal electron flux derived from the hard X-ray spectroscopic analysis. For the first time, the shear is inferred using several complementary techniques allowing for a cross-validation of the results. The measurements confirm the strong-to-weak shear evolution proposed in previous observational studies, which inferred the shear from the foot-points, and reproduced in numerical models (Dahlin et al. 2022). A key finding is that, in this flare, reconnection is not an efficient producer of energetic non-thermal electrons during the first ten minutes when the strongly sheared PRFLs are formed. We suggest that an intermediate shear angle below 40 degrees is needed for efficient particle acceleration via reconnection, and we propose a theoretical interpretation.