One-dimensional values of the field line angular velocity (top) and the toroidal magnetic field (bottom) for the Wald test (Cartesian CARPET grid) at t = 256 M_⊙ and using different BH dimensionless rapidities (see legends). The interpolation radius (for the extraction in a Cartesian grid) is indicated in the respective panel, corresponding to the ergosphere radius at the equator (top) or the BH horizon radius (bottom).