HMI vector magnetograms showing the evolution of HARP 1367 at three selected times corresponding to Fig. 2 shown in panels a–c. The background greyscale image shows the vertical field (B_z), and the overplotted arrows represent the horizontal fields (B_x, y) with color indicating the field strength. The vertical orange line in each panel marks the X position of the moving pore. Panels d–f: magnetic field diagnosis for the local area of moving pore. The horizontal field in the rectangular coordinate (B_x, y) has been transformed into a polar coordinate (B_r, θ) with the center of the pore as the origin. The region of interest has been divided into two parts: the area moving frontwards and that moving behind, as illustrated by the two white fan-shaped areas in panela. Azimuthally averaged components of the magnetic field vector (B_r, θ, z) are calculated as a function of the radial distance r from the pore center as shown by the colored curves in three panels. The positive and negative directions of the x-axis represents the radial distance in front of and behind the moving pore, respectively.