a) Magnetic field lines computed from the NLFFF model. Different colors indicate different field line systems. The background is the vertical magnetic field B_z shown with gray levels. White/black color represents positive/negative magnetic polarity. The box marks the field of view shown in panels b)–f). b), c) Orange ribbons represent the QSL sections on the photosphere. Only the QSLs with the squashing degree Q ≥ 10¹⁴ are plotted. Gray/black contours are the same as that in Fig. 4c. d), e) Magnetic field lines overlaid on the photospheric distribution of the vertical electric current density j_z as shown in Fig. 4c. The current j_z is computed with the non-preprocessed vector magnetic field. f) Similar to panels d) and e), but only some sample field lines with magnetic dips are shown.