k − ω diagrams of the LoS velocity (panel a) and CP (panel b). The dashed white line represents the acoustic theoretical fundamental mode (gk, where g is the gravitational acceleration and k the horizontal wavenumber), while the continuous black line corresponds to the Lamb line and shows the theoretically expected propagation at the sound speed (c_sk, where c_s is the sound speed). Panels c and d: RMS amplitude of the filtered velocity and CP, respectively. Panels e and f: filtered velocities and CP signals, respectively, at one instant in time. The filter widths (in both k and ω space) are illustrated by the dashed black boxes in panels a and b, where most of the power is located.