SOLARNET Conference: The Many Scales of the Magnetic Sun

Bald patch (BP) is a magnetic topological feature where U-shaped field lines graze the photosphere. Previous studies often found them in emerging solar active regions (ARs) with patchy flux distribution, or decaying ARs with weaker fields. Here we study the evolution of kilogauss BPs in the delta-penumbral light bridge in AR 12673. The gradual formation spanned ~16 hours, which was accompanied by increasing photospheric Doppler velocity. HMI and SOT-SP data suggest that the large Doppler signal mostly comes the projection of the field-aligned Evershed flow. Significant line-of-sight gradient is likely present in the magnetic field vectors. During the subsequent GOES X9-class flare, HMI data reveal fast destruction of the BPs over ~30 minutes. The horizontal magnetic field component perpendicular to the polarity inversion line reversed its direction, while the parallel component exhibited a ~1kG increase. The observations suggest that magnetic reconnection may involve entire BP separatrices and change the magnetic topology from BPs to sheared arcades.