Power spectra P(k) of the Doppler velocity as a function of k. The curves here are for the same datasets as in Fig. 2 and with the same color and line style criteria. Left: spectra for the observational data and for the HD run. Right: the four simulated runs and the corresponding portion of the second IMaX time series (solid black line). The straight lines in the left panel show power laws ∝ k² (dashed line), ∝ k^−17/3 (dash-dotted) and ∝ k^−29/3 (solid).