Scatter plot of the density of points in the plane (β_e, γ) for the N = 631 samples selected by the data set of the Cluster spacecraft. A power law ﹩{\beta }_{e}^{-3/2}﹩ is reported as a dashed red line. The 400 Hz noise floor (Alexandrova et al. 2012) is marked by a blue dashed horizontal line: points corresponding to γ values above (below) it are displayed as open (filled) signs. The γ values have been obtained for all samples from best fits of Equation (3). For each γ, the corresponding value of β_e is measured from the parameters of the sample. In the inset figure we plot the plane (β_e, γ) by using only a density of points ≥20.