Emergence and cancellation of magnetic flux in HMI intensity images and magnetograms. A and B represent the emerging pores, and the cyan, blue, and green arrows show the emerging magnetic polarities (P1, P2, and N3). The dashed box indicates the magnetic interaction region for P1–P2 and N3, and the positive (blue) and native (green) magnetic flux changes in this region is shown in panel i. The blue and green arrows indicate the trough of the positive flux curve and the peak of the negative flux curve, respectively. The dotted lines and two dashed lines represent the timings of two brightenings and the eruption onset, respectively. The temporal evolution of magnetic fields and pores is shown in the online movie of Animation 1.