Illustration of the evolution of magnetic field and plasma in the test simulation with a potential field. a) and c) maps of log ₁₀(ρ [g/cm³]) in the (r,φ) plane passing through the centre of the bipoles (at t = 0 τ_Alf, and at t = 142.10 τ_Alf). b) and d) same maps at the r = 1.09 R_⊙ surface. Superimposed are magnetic field lines plotted from the same starting points (green lines) and the contour line of β = 1 (white line).