Maps of log₁₀(ρ [g/cm³]) in the (r−φ) plane passing through the centre of the bipoles at different times for different test simulations. Superimposed are magnetic field lines plotted from the same starting points (green lines) and the contour line of β = 1 (white line). a) and d)T = 3 MK, B_max = 7 G; b) and e)T = 2 MK, B_max = 21 G; c) and f)T = 1.5 MK, B_max = 42 G.