Contours of the field-line-integrated parallel electric field, plotted where field lines intersect the mid-plane. Times shown are (a) t = 175, (b) t = 200, (c) t = 225, (d) t = 275, (e) t = 325, and (f) t = 400. Times (b), (e), and (f) are subsequently considered in greater detail. In (a), the dashed black lines indicate the radii of the driven regions, representing footpoints of the left, central, and right threads. The contours are shaded according to the logarithmic scale on the colour bar.