Panels a–b: Top view and side view of the 3D coronal magnetic field reconstructed with the potential-field extrapolation; panel c: Distribution of the exterior angle (color scale) overlaid on the SDO/AIA 193 Å map at 03:15:04 UT; panels d–f: Selected field lines, indicating those field lines traced from the western (yellow) and northern (magenta) regions. These are more backward inclined compared to those from the eastern (cyan) and southern (green) regions.