Expansion factors (a) at the front and (b) at the rear MC boundaries as a function of the magnetic asymmetry coefficient c_B, x, i.e. Eq. (24) applied to the magnetic data. The results of linear (blue) and quadratic (red) fits for a given MC are shown with two nearby points located on the same vertical line. The model fit derived from Eq. (21) (in green in earlier figures) is omitted because the results are similar. The Pearson (P) and Spearman (S) correlation coefficients are reported at the top of each panel for linear velocity fits. The ageing effect alone is expected to create a magnetic asymmetry that is correlated with the expansion factors, so that for asymmetry due to ageing, data points are expected to be included in the yellow regions.