Top view of the magnetic structure of AR 11928 at four different times of the evolution. Color scale along each field line represents the horizontal field strength. The lower boundary (z = 0) shows the vertical component B_z at the photospheric level with gray levels in the range [−100, 100] G. The white rectangular box in panel a) refers to the field of view of panels in Fig. 1 and the black dashed rectangular box indicates the field of view of observations as plotted in Fig. 6.