Simulation with T = 2 MK and B_max = 21 G. a)–c) Maps of density contrast (Eq. (10)) in the (r−φ) plane passing through the centre of the bipoles at t = 0 h, t = 0.58 h, t = 1.74 h. Superimposed are magnetic field lines plotted from the same starting points (green lines) and the contour line of β = 1 (white line). d)–f) Maps of B_θ/|B | on the same plane and at the same times. Maps show the full domain of our simulations from r = 1  R_⊙ to r = 4  R_⊙. In panel d) the yellow dashed line is the cut for the plots in Fig. 6. The temporal evolution is available in the on-line edition.