Measuring and modeling the shear evolution of post-reconnection flare loops

Observations of solar flares have suggested a strong to weak shear evolution of post reconnection flare loops (PRFLs). Previous work has inferred shear evolution from the foot-points of PRFLs. In this work, we quantify magnetic shear by calculating the complement of the angle formed between the PRFLs (projected in the image plane) and the photospheric magnetic polarity inversion line (PIL) where the radial magnetic field switches sign. The evolution of magnetic shear is evaluated for both observed PRFLs and modeled PRFLs anchored at flare loop foot points. The two-ribbon flare SOL20141218T21:40 observed by the Solar Dynamics Observatory is investigated. We have identified more than 1000 PRFLs observed by the Atmosphere Imaging Assembly, and found an average shear angle that decreases from over 40 degrees to less than 20 degrees in about 10 minutes. We then model the PRFLs using extrapolated linear force free field. The modeled loops are traced from the observed foot points in the UV 1600 passband, and compared with observed loops in EUV passbands. We find that the force free parameter must decrease throughout the flare evolution for the modeled loops to resemble the observed loops. We also compare the temporal and spatial distribution of angles formed by observed and modeled PRFLs. This study confirms the strong-to-weak shear evolution in this flare, and has improved measurements of magnetic shear by using observations and models of PRFLs.