Streamline diffusivity for seed points placed within spheres of the indicated radii at 200 kyr (left) and 390 kyr (right). Triangles, circles, and squares indicate clusters I, II, and III, respectively. The near linear increase for the 5 pc seed cloud radius is due to quasi-radial field-line bundles embedded in transonic sheets (cf. Fig. 5, lower panel) and the levelling-off is due to fieldlines travelling close to the superbubble edge looping back to the core. The variation between clusters I and II, which only differ in the spatial distribution of stars, illustrates the importance of local wind interactions.