Rapid Evolution of Bald Patches in a Major Solar Eruption

TL;DR

This study investigates the rapid evolution and disintegration of bald patches in solar active regions during a major X9.3 flare, revealing magnetic topology changes associated with reconnection processes.

Contribution

It provides new observational evidence of how bald patches evolve and disintegrate during intense solar eruptions, linking magnetic topology changes to flare dynamics.

Findings

The central bald patch disintegrated within 35 minutes.

Magnetic shear angle rotated by 9 degrees during disintegration.

Significant step-wise increase in horizontal magnetic field component.

Abstract

Bald patch (BP) is a magnetic topological feature where U-shaped field lines turn tangent to the photosphere. Field lines threading the BP trace a separatrix surface where reconnection preferentially occurs. Here we study the evolution of multiple, strong-field BPs in active region 12673 during the most intense, X9.3 flare of solar cycle 24. The central BP, located between the initial flare ribbons, largely "disintegrated" within 35 minutes. The more remote, southern BP survived. The disintegration manifested as a 9-degree rotation of the median shear angle; the perpendicular component of the horizontal field (with respect to the polarity inversion line) changed sign. The parallel component exhibited a step-wise, permanent increase of 1 kG, consistent with previous observations of the flare-related "magnetic imprint". The observations suggest that magnetic reconnection during a major eruption may involve entire BP separatrices, leading to a change of magnetic topology from BPs to sheared arcades.