Comparison of the asymmetry coefficient c_B, x without the ageing effect (ordinate) as a function of c_B, x derived from the uncorrected magnetic data (abscissa). (a) The results of linear and quadratic fits of the observed velocity are very similar. The points with a lighter colour are in compression (ΔV_lin >  0, see Fig. 2a). (b) The results of the model fit, Eq. (21), of the observed velocity are almost the same as the results with the quadratic fit. The diagonal black line is c_B′,x′ = c_B, x. The Pearson (P) and Spearman (S) correlation coefficients are reported at the top of the panel for linear and quadratic velocity fits.