Field lines traced from each driven boundary, z = ±L. Here, the field lines drawn at t = 180 (panel a), t = 230 (panel b), t = 310 (panel c), and t = 450 (panel d), corresponding with the times in Fig. 5, are visible. The darker field lines are traced from the bottom boundary, and the lighter from the top. These are produced from a lower resolution, 256² × 1024, because of limits on computational resources.