Isosurfaces of local E_∥. Times shown are (a) t = 175, (b) t = 200, (c) t = 225, (d) t = 275, (e) t = 325, and (f) t = 400. On (a) is superimposed a sample of field lines in green. The radii of the left, central, and right driving regions are marked on the bottom z = −L plane. Times (b), (e), and (f) are subsequently considered in greater detail. The isosurfaces are shaded according to the logarithmic scale on the colour bar.