Correlation ratio modelled with Gaussian spatial variations of the polarized dust spectral index, in LR42 as in Fig. 2. The HM and DS Planck measurements are shown as diamonds and squares, respectively. The model is plotted for four values of , which is defined as the standard deviation of the Gaussian realization of the spectral index after smoothing at 1°. The first model, with , is described in Sect. 3.2.