Locations on z = L to which field lines traced from z = −L connect. These are shown for t = 180 (panel a), t = 230 (panel b), t = 310 (panel c), and t = 450 (panel d), corresponding with the times in Fig. 5. Field lines in blue begin in the central thread, those in orange in the left, and those in purple in the right. These are produced from a lower resolution, 256² × 1024, as for Fig. 6.