When Magnetic Field Lines Stretch, Snap, and Expand: A New Look at Solar Flares with L-maps

TL;DR

This study introduces L-maps as a new diagnostic tool to analyze the three-dimensional evolution of coronal magnetic fields during solar flares, combining simulations with observations for improved understanding.

Contribution

The paper presents L-maps for tracking magnetic field-line dynamics during flares, validated by observations, and identifies distinct stages of coronal evolution using clustering analysis.

Findings

L-maps effectively identify flare ribbons and coronal dimmings.

Strong agreement between simulated L-maps and SDO/AIA observations.

Three distinct stages of magnetic evolution during flares are distinguished.

Abstract

Understanding the three-dimensional evolution of coronal magnetic fields during solar flares remains challenging due to the lack of direct coronal field measurements. Here we combine data-driven MHD simulations of NOAA AR 11158 (Fan et al., 2024) with flare-ribbon and coronal-dimming observations to investigate realistic coronal magnetic-field evolution during an X-class flare. We introduce L-maps - maps of natural logarithm of magnetic field-line lengths - as a diagnostic tool to track the dynamics of simulated coronal magnetic structures. Variations in L-maps identify flare ribbons through field-line shortening and coronal dimmings through field-line lengthening. Comparison with SDO/AIA observations demonstrates strong morphological and temporal agreement, validating the simulated field evolution. Applying K-means clustering to the L-map temporal profiles, we distinguish three stages of coronal evolution: (1) slow pre-flare rise phase, (2) flare reconnection accompanied by CME rise, and (3) post-reconnection CME expansion. We detect a slow pre-flare rise phase of magnetic field lines rooted in ribbon footpoints and identify reconnection dimming - area of rapid expansion of active-region core magnetic field lines during flare impulsive phase due to reconnection. Our results show that L-maps provide a powerful and physically intuitive framework for bridging simulations and observations and for tracking the full three-dimensional evolution of coronal magnetic fields during flares.