Initial condition of the simulations. Top left: synoptic magnetogram corresponding to CR 1625. Top right: synoptic magnetogram corresponding to CR 1625, remapped to an equiangular grid and resampled to the highest angular degree order, l, used to compute the potential field extrapolation. Bottom left: radial integral of the azimuthal field across the convection zone, b_φ, extrapolated from the surface field. Bottom right: radial integral of the θ component of the magnetic field across the convection zone, b_θ, extrapolated from the surface field. Red and blue indicate opposite polarities in all the maps.