Comparison of spectral indices of dust polarized emission and total intensity. The spectral indices are derived from the 353-to-217 GHz colour ratio. Plots to the left show the results obtained from our simulated maps, and the ones to the right are from the Planck data. Distinct symbols are used to represent each of the six sky regions, as in Fig. 10. For the simulations, the dashed lines represent the input dust spectral indices (, ). For the data, the dashed lines represent the mean measured dust spectral indices (, ).