Selected field lines belonging to the three QSLs of NOAA 11589 identified in Fig. 6b, for the same extrapolations as in Fig. 7. Red/orange, dark/light-blue, and green field lines respectively belong to Q₁, Q₂, and Q₃. The gray scale displays the photospheric map of the squashing degree, Q. Solid purple/cyan lines are isocontours of the photospheric vertical magnetic field, B_z = [ 150,300,600 ] Gauss.