Sketch representing the N-S asymmetry of the solar magnetic field in terms of magnetic fluxes through the polar caps. The polar caps on the solar disk at the distance of 1 R_⊙ ideally represent regions in polar coronal holes with equal area, which depend upon the half-opening angle θ. Because of the asymmetric over-expansion of the magnetic field lines between the two hemispheres (represented by the curved dashed lines departing from the solar disk), the angle θ at a given distance r increases by a factor a_N in the north and a_S in the south, with a_N>a_S. The resulting size of the northern polar cap is larger than the southern cap.