Spherical harmonic structure of the photospheric magnetic field. (a) Normalized zonal Gauss coefficients of the photospheric magnetic potential for three consecutive solar minima. The two-dipole model obtained from the first four Gauss coefficients is shown in black. (b) Normalized zonal spatial power spectra of the photospheric magnetic field for the same solar minima as in panel a, showing the relative contributions of the different harmonics (multipoles) to the total power.