a)−d) Maps of log ₁₀(ρ) on the (φ,r) plane passing through the centre of the bipoles at different times. Super-imposed are magnetic field lines plotted from the same points (green lines) and a contour line at β = 1 (white line). e)−h) Magnetic configurations at the same times as the LHS column. Yellow magnetic field lines depart from the negative polarity of the LHS bipole, blue magnetic lines depart from the positive polarity of the LHS bipole, green magnetic lines represent the external magnetic field. All the magnetic field lines in panels a)−d) and e)−h) are plotted from the same starting points at all the times.