Observation and Modeling of Shear Evolution of Post-reconnection Flare Loops

Abstract

A solar flare releases magnetic energy by reconnecting field lines across a current sheet, thereby allowing their relaxation to a lower-energy state. The maximum possible energy is released if all field lines relax to a current-free (potential) state. The progress of a flare’s reconnection is often measured as the angle-complement between the observed post-reconnection flare loops and the polarity inversion line of the photospheric magnetic field: shear angle. Many observations have shown strong-to-weak shear evolution over the course of a flare. A field line’s shear angle is, however, an imperfect measure of its relaxation. We develop a new technique for observationally inferring the 3D structure of post-reconnection field lines, including their local twist, α , which will vanish for potential fields. Our method fits loops in extreme-ultraviolet (EUV) images to extrapolations subject to constraints such as matching the feet of model field lines to observed flare ribbons. We apply the new method to an eruptive two-ribbon flare (SOL2014-12-18T22), which exhibits strong-to-weak shear-angle evolution. We find that, as the flare progresses, α decreases in post reconnection loops anchored to newly brightened ribbons. Our study demonstrates that post-reconnection magnetic field is neither potential nor linear force-free. The method quantifies, for the first time, the time-history of a flare’s energetic relaxation. It also quantifies the increasing height of the subsequently reconnected field, and the time delay between reconnection forming a flare loop and its appearance in EUV passbands. These results promise to enable improvements in both magnetic modeling and hydrodynamic modeling of flares.