Colour coded figures for the azimuthal component of the mean magnetic field at time evolution from left to right: 11.7, 12.5 and 12.9 Gyr. Fields are averaged over lengths 1 kpc. The top row shows model 75, the lower model 76. The continuous contours delineate the regions (“arms”) where field is injected; these rotate rigidly, with pattern speed such that the corotation radius is at fractional radius r ≈ 0.7.