A hypothetical magnetic field (Appendix A) whose field line mapping matches that inferred from the observed velocity field. The magnetic field lines in this particular field are simply trajectories of tracer particles in the 2D, time-dependent velocity field (with time increasing vertically). Contour slices show B_z at different z with the colour scale in Gauss: the initial distribution (on the lower boundary) is a uniform field B_z(x,y,0) = 88   G, which was the average field strength in this region of the original SOT magnetogram.