Observed (top) and fitted (bottom) Q (left) and U (right) Stokes parameter maps in orthographic projection, in units of MJy sr⁻¹ . Each pair of maps shows the sky divided into its two Galactic hemispheres: north (left) and south (right). The vertical full line in each map indicates ℓ = 0^°. The grey regions correspond to the areas that are masked. The texture seen in the model maps, overlaid on the regular patterns, arises from the intensity I. Planck observations show additional structure from the turbulent component of the magnetic field, not included in our model.