(a) SOHO/MDI magnetograms of AR 9748. The magnetograms have the same contours and colour convention as Fig. 7a. (b) Evolution of the AR magnetic flux computed from the magnetograms (black line). (c) Evolution of ϕ_a derived from the magnetic barycentres (black line) and ϕ_c obtained from the core flux centres with the same coloured pattern and input parameters as the ones used in Fig. 7c. The associated movie is available online (fig9_a.avi).