AIA image at 1600 Å of the same AR region shown in Fig. 3 saturated at the intensity of 1180 and overplotted with the contours at the same value at the beginning of the X2.2 flare of AR 11158 (top). Zoom of the vertical magnetic field 2D map at the same time, with the two regions of interest (bottom). Red contours are the same as in the top panel, only rebinned to the resolution of the magnetic field image. Yellow contours correspond to the magnetic field value B_z = 0 G.