Evolution of the magnetic fileds. Panel a: HMI LOS magnetograms superimposed over the velocity fields. The blue (orange) arrows represent the velocity of the negative (positive) polarities. Panel b: temporal evolution of integrated magnetic fluxes and normalized 304 Å and 171 Å intensities. The black box shows the region for integrating magnetic fluxes. The shadowed regions correspond to the time periods of bursts shown in Fig. 5.