Background radial magnetic field during solar minimum estimated from the data series hmi.b_synoptic (Liu et al. 2017). The synoptic maps for the radial component of the magnetic field were stacked in time and then Gaussian-smoothed with a full width of 27 days. The red curve shows the data in the northern hemisphere on 15 September 2019, when the Sun was quiet and its north pole was tilted toward the Earth. The black curve shows the approximation B_0r(θ) = 10 × (cos θ)⁷ G, which we use in Eqs. (B.5) and (B.6).