Two-dipole model. (a) Two rings of surface dipoles representing the north–south magnetic component of decaying active regions in the photosphere at the latitude of about ±30° in an axisymmetric case. (b) Two axial dipoles placed at the center of each dipole ring of panel a. The magnetic potential of the two dipole rings of panel a and of the two axial dipoles of panel b are nearly identical at the solar surface.