Coordinates for the 1D FFMD simulation on the equatorial plane of the spinning black hole. We assume that the electromagnetic field is force-free, axis-symmetric, and plane-symmetric with respect to the equatorial plane and that the magnetic surfaces are fixed to radial. As the black hole is not a magnetic monopole, the radial component of the magnetic field should be negative (opposite sign to that of the component at the equatorial plane) outside the calculation regions, e.g., those around the polar regions.