Solar active region NOAA 11158 at ~06:28 UT on 2011 February 14. a) Photospheric vertical magnetic field (B_z) in grayscale overplotted with the flux transport velocity field (blue/green arrows) and the polarity labels (red). b) 3D views of the NLFFF magnetic field extrapolation with selected magnetic field lines. Each field line color corresponds to a different quasi-connectivity domain (separated by QSLs; see Sect. 4). The orange arrow shows the viewing angle relatively to panel a). The magnetic field values are saturated at ±1500 Gauss. Solid pink and dashed cyan lines are ±500 Gauss magnetic field isocontours, respectively.