Observation image of the HMI M45s longitudinal field. The region surrounded by red lines in panels a and b is the positive polar magnetic field evolution region 1 determined by the magnetic field value at 20:30:06 UT of +25 Gauss, and the blue line is the negative polar magnetic field evolution region 2 determined by the magnetic field value at 21:51:16 UT of –120 Gauss. The yellow and green box represent the rectangular region in which the positive and negative magnetic field evolution region, respectively, is located.