Evolution of the polar surface magnetic field strength as a function of time for the same models as those shown in Fig. 7. Models with P_rot, ini = 1, 10, 100, and 1000 d correspond to red solid, dashed, dotted, and dash-dotted lines, respectively. Models with B_p, ini = 10 G are shown in the top left panel, those with B_p, ini = 100, 1000, and 10 000 G are, respectively, shown in the top-right, bottom-left, and bottom-right panels. The black solid lines depict models without the η effect with P_rot, ini = 1000 d for each initial magnetic field strength.