Filled green contours: density profile at z = 0 Mm. The colour table used for the density is the same as in Figs. 5, 11 and 13. Line contours: value of Ξ^* (see Eq. (19)) for field lines embedded within the Kelvin-Helmholtz mixing layer. The time shown is t ≈ 1090 s for the straight field, high resolution, l = 100 Mm simulation. We have normalised using the maximum value of Ξ^* at this time. For clarity, we have restricted the x axis in each panel to −1.2 Mm ≤ x ≤ 1.2 Mm.