Double-decker flux rope configuration above the PIL in the AR core region revealed by NLFFF modeling at 11:24 UT on 2017 September 6. Panels a–b: top view and side view of two flux ropes (FR1 and FR2) composing the double-decker configuration. The FOV of these panels is approximated by the white square in Fig. 1d. Panel c: isosurfaces of twist number T_w = –1 (white) and T_w = –1.75 (red) viewed from the same perspective as panel a. Panel d: twist number distribution in the vertical (x–z) plane along the green cut labeled in panel c.