Mean magnetic field strength as a function of time in various models. In all frames, the fiducial model is shown by a black solid line. Other models are the following: a) blue dashed line, model without imposed spirals; b) models with a different pitch angle 10° (red dashed line) and 30° (blue dash-dot line); c) models with a different arm number m = 3 (red dashed line) and m = 4 (blue dash-dot line); d) models with a different initial magnetic field β = 1 (red dashed line) and β = 0.3 (blue dash-dot line).