Evolution of the tilt angle for non-uniform twist models with N_t, 0 = 0.5 and (a) h = −1, g = 0, (b) h = 0, g = 0.8, and (c) h = 0, g = −0.8. The black line shows the apparent tilt angle estimations obtained from the polarity barycentres, ϕ_a. We plot ϕ_c computed with CoFFE without (green) and with convergence (red) achieved (see inset). The red-shaded area corresponds to the estimation of ϕ_c computed with different values of p within the interval [−1, 1] and after achieving the convergence criterion of the tilt angle described in Sect. 2.2. The associated movies are available online (fig11_a.avi, fig11_b.avi, and fig11_c.avi).