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

TL;DR

This paper introduces a new observational technique to analyze the three-dimensional magnetic structure of post-reconnection flare loops, revealing their relaxation process and providing insights into flare energetics and magnetic field evolution.

Contribution

The study develops a novel method for inferring 3D magnetic field structure in flare loops, including local twist, from EUV images, enhancing understanding of magnetic relaxation during flares.

Findings

Post-reconnection loops show decreasing twist, indicating relaxation.

Magnetic fields are neither potential nor linear force-free during flares.

Quantifies the timing and height evolution of reconnected field lines.

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 three-dimensional structure of post-reconnection field lines, including their local twist, $\alpha$, which will vanish for potential fields. Our method fits loops in 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, $\alpha$ 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 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.