Streamlines of the magnetic field coloured by magnitude. In the upper panel, streamline seed points are placed within the radius of the star cluster. This highlights the tangled morphology arising from the interaction of Parker spiral stellar magnetic fields. The lower panel shows streamlines for a seed point radius comparable to the size of the cluster-wind termination shock (5 pc) over-plotted onto the surface where the sonic Mach number equals three. The surface colour shows the orientation of the flow velocity, u, with respect to the surface normal, n. We note the quasi-radial streamline bundles, which are dragged outward by coherent flows. Such flows are present, for example, in transonic sheets (see Fig. 3b). The figure shows cluster I at 390 kyr.