3D magnetic configuration of the FC. Panel a: AIA 304 Å image with the dotted curve overlaid showing the path of the FC. Panel b: collection of 3D magnetic field lines indicating the MFR as seen from the top. The bottom boundary is the HMI LOS magnetogram. Panel c: side view of panel b coupled with Q values. L1 and L2 delineate two groups of sheared arcades, and M1 and M2 represent the MFR field lines and small flare loops, respectively. The coordinates x, y, and z represent west, north, and altitude. Panel d: distribution of Q values on the plane perpendicular to the MFR axis, as pointed out by the dashed line in panel b. Panel e: distribution of J/B in the same plane and the dotted line indicates the outer boundary of the MFR.