Top panel: photospheric distribution of squashing factor Q, with the bald patch locations (green dots) overlaid. Middle panel: photospheric squashing degree, with bald-patch separatrix field lines and isocontours of B_z overlaid. The bald-patch separatrix field lines are traced from the bald patch locations (green dots) shown on the top panel. Bottom panel: side view of the bald-patch separatrix lines.