Magnetic field strength estimated from the NLFFF model. Panel a: zoomed view of the longitude–latitude map of the radial component of the photospheric magnetic field by SDO/HMI in the HIRES region at 16:36 UT on 2014 September 6. The blue curve with circles at the two ends refers to the path along which we insert the flux rope. Panel b: selected field lines from the NLFFF model overlaid on a flare image by AIA, and the red (green) contours refer to the positive (negative) photospheric magnetic fields by HMI. Panel c: the distributions of the magnetic field strength from this model with height at three locations, L1, L2, and L3 (marked with yellow stars on panel a.