Evolution of magnetic field configurations. Left-hand column is model 75, middle column model 76, right-hand column model 77. Models (a), (b), (c) (first row) at approximate time 11.7 Gyr; models (d), (e), (f) (second row) at time 12.5 Gyr; models (g), (h), (i) (third row) at 12.9 Gyr; models (j), (k), (l) (bottom row) at approximately 13.2 Gyr (i.e. “now”). For model 76 the rotation parameter R_ω has been increased from that of model 75, and in model 77 the magnitude of the injected field, B_inj0, has been decreased from that used in model 75. 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.