Three-dimensional coronal magnetic field of the active region. Panels (a) and (c) show the nonpotential structures around the central PIL at 01:36 and 02:36 UT, overlaid with the B_z map. The white straight lines mark the locations used to calculate the poloidal magnetic field above the nonpotential structures. (b) Comparison of the poloidal magnetic field distributions before and after the flare. The colored solid lines denote the contours of the poloidal magnetic field at 01:36 UT, whereas the colored dashed lines show the distributions at 02:36 UT. (d) Schematic diagram of the magnetic reconnection process in this event. The blue arrowed lines indicate the sheared magnetic arcades before magnetic reconnection. The green and orange arrowed lines represent the flare loop and twisted magnetic field line formed after the reconnection, respectively.