Projected 2D view of the squashing degree Q (left panels, see Sect. 3) and of the decay index (gray color scale, right panels) along a plane passing through x = 435 (see Fig. 7). The HFT corresponds to the intersection of the two high Q regions in the left panels. The green, cyan, and red contours on the right panels indicate isocontours of decay index n = 0.5,1, and 1.5, respectively. The yellow contour indicates the polarity inversion line. Axis units are in Mm from the bottom left corner of the larger remapped HMI/LOS magnetogram used to perform the potential field extrapolation (resp. from the photosphere) for the abscissa (resp. the ordinate).