Amplitude of the dust A^EE (red squares) and A^BB (blue circles) power spectra, normalized with respect to the largest amplitude for each mode. These are plotted versus the mean dust intensity ⟨I₃₅₃⟩ for the six LR regions (top panel). A power-law fit of the form A^XX(⟨I₃₅₃⟩)= K_XX⟨I₃₅₃⟩^1.9, X ∈ { E,B }, is overplotted as a dashed line of the corresponding colour (these almost overlap). The bottom panel presents the ratio of the data and the fitted ⟨I₃₅₃⟩^1.9 power law; the range associated with the ± 1σ uncertainty in the power-law exponent of 1.9 is shown in grey. For details see Sect. 4.3.