(a) ρ − σ² scatter plot for UVCS O VI observations performed between 1.3 and 2.8 R_⊙ (different colors refer to different distances as reported in the legend); the corresponding values of the Spearman's rank correlation factor r are also shown. (b) Radial profile of the correlation factor r fitted with a logarithmic function (red curve). (c) Intensity ratio ρ as a function of the UVCS observation distance (colors are the same as used for panel (a)); the corresponding mean values and error bars (inferred from the standard deviation) are also displayed; ρ = 0.5 is marked by the dashed line; the sonic point r_c ≃ 1.9 R_⊙ is indicated by the dotted line. (d) ρ − σ² scatter plot in the subsonic (blue) and supersonic (red) regions, separated by the ρ = 0.5 dashed line. (e) Radial profile of the amplitude of the ρ (red) and σ² (blue) fluctuations, normalized to the corresponding average values; the dashed line marks the sonic point r_c, separating regions of subsonic and supersonic flow.