Magnetic field evolution. Panels (a1)–(h1) present the HMI magnetograms of the main region, where the brightening events occur. Panels (a2)–(h2) are the same as panel a, but overlaid with ±50 G magnetic field contours, facilitating the identification of magnetic polarity motion, emergence, and cancellation. Red and blue contours represent positive and negative magnetic polarities, respectively. Panels (i)–(k) show the temporal evolution of the total magnetic flux, positive magnetic flux, and negative magnetic flux of the region in panel a1.