Radial profiles of the horizontal velocity of the flow system of AR11646 for six successive days (panels a–f), representative for Case (i) of the evolution of the flow field. Eight time steps are equally distributed across one day (colour-coding from blue, meaning early, to red, indicating late). The penumbra has dissolved on the the fourth day of the analysis at time step 4 (turquoise line in panel d). Error bars represent the standard deviation. Vertical lines display the position of the spot boundary.