Above: field lines traced from fixed locations on the lower boundary (and coloured according to location on the lower boundary). Below: locations of intersection with the plane z = +24 of field lines traced from a regular grid in the plane z = −24. Field lines traced from locations x ≥ 0 on the lower boundary are coloured white, from x < 0 black. Plots are made for t = 0 (left) and t = 290 (right) for the run with η = 10^-3.