Overview of the surface magnetic field. (a) PHI line-of-sight (LOS) magnetogram showing the magnetic field distribution of the associated main active region before the eruption. (b) 3D magnetic field configuration, traced by the magnetic field lines, showing the initial condition of data-constrained 3D MHD simulation. The background image is the zoom-in of the PHI LOS magnetogram, as marked by a blue square in panel (a). (c) Same as panel (b), but for the side view and with a small panel more clearly showing the flux rope.