(a) SOHO/MDI LOS magnetograms of AR 9574. The red- and blue- shaded areas represent the positive and the negative B_z magnetic field component. The magenta contour in each map corresponds to the field magnitude of ±50 G. The black circular contours are the half-width level of the Gaussian fit for each polarity using p = 0 and n_it = C_it. The green and blue isocontours indicate the areas where the field is larger and lower than the Gaussian, respectively, by 50% of the maximum difference between the Gaussian and the local observed field. The black and the magenta segments show the tilt of the AR respectively obtained with the core flux centres (which define ϕ_c) and the magnetic barycentres (which define the apparent tilt ϕ_a). (b) Evolution of the AR magnetic flux computed from the magnetograms. The vertical dashed line marks its maximum value. (c) Evolution of ϕ_a derived from the magnetic barycentres (black line) and ϕ_c obtained with CoFFE (see inset). The red-shaded area corresponds to the values of ϕ_c computed for p within the interval [−1, 1] after convergence criterion of the tilt angle is achieved. The associated movie is available online (fig7_a.avi).