a) Normalized histogram of the seismologically estimated l/R for three values of the flow velocity (indicated within the panel) and a maximum background noise corresponding to 50% of the initial amplitude. b) Same as panel a) but with u₀/c_Ap = 0.1 and three percentages of maximum background noise (indicated next to the lines). In both panels, the dashed lines are the Gaussian fits and the vertical dotted line is the actual l/R.