Dependence of Π on the local spectral index α, for s = 2 and various levels of the isotropic random fields. The choice of the levels is the same as in Fig. 1: it starts with ${log}_{10}\sigma /\overline{B}=-0.5$, and then increases in steps of 0.25. Π decreases with increasing $\sigma /\overline{B}$, and for values of ${log}_{10}\sigma /\overline{B}>0.5$ the curves can no longer be distinguished from zero in the plot. The dashed lines refer to cases with β = 1, while the solid lines to β = 2. The corresponding curves are almost superimposed, which means that these correlations are very insensitive to the choice of β.