Changes of photospheric magnetic field across the MFR eruption. Panels a1–a3 and b1–b3: changes of inclination θ, radial component B_r, and horizontal component B_h of photospheric magnetic field after the first and second episode of ribbon sweeping, respectively. The PROI is outlined by the closed curve. The time-distance maps in panels c–e are made by applying the linear slit (Fig. 2d) to HMI vector magnetograms subtracted by a reference magnetogram at 13:00 UT. The purple and green bars on the top of panels c–e indicates the two episodes of ribbon sweeping during the rope-buildup and rope-erosion stage, respectively. δθ is color-coded by the color bar of the same as in panels a1 and b1.