Surfaces of constant sonic Mach number visualising (a) the cluster-wind termination shock and (b) transonic sheets. The two panels in (a) show the orientation of the magnetic field, B (left), and the flow velocity, u (right), with respect to the Mach surface normal, n. In the latter plot, cones (blue, dominated by perpendicular flow) can be clearly distinguished from sheet base shocks (red, inbetween adjacent cones) and coupled stellar winds (red, in the centre of cones). In panel b, the M_S = 1 surface is over-plotted onto the M_S = 3 surface surface in transparent white, highlighting that sheets extend outward from regions of parallel flow. The panels show cluster I at 390 kyr.