Dependence of the coherent PDS radial length scale (left panel) and power spectral density (right panel) on solar wind proton density. Grey dots correspond to the data and the solid black lines to the median of each n_p bin, respectively, while the shaded grey area is the inter-quartile range. The power law fit on the data is depicted with the solid red line, while the root mean square error (RMSE) is given at the top of each panel.