Sketch of the bipolar configuration. Potential magnetic field lines (pink curves with arrows) and two magnetic surfaces associated to the EPs α (green color) and β (brown color) for a bipole with d = 0.25 and z₀ = 0.5 (all the lengths are hereafter normalised to H). The intersection of the two surfaces, see the yellow curve, represents a particular magnetic field line. The distribution of the vertical component of the magnetic field at z = 0 is represented in gray colors.