2D velocity cross-amplitude power Q^(0.2, 0.5),  Q^{(0.2, 0.75)}, and Q^(0.2, 1.0), derived from the corresponding velocity cross spectra, at the same time step as in Fig. 2. The cross spectra are obtained by correlating the velocity measurements at the line bisector level 0.2 and the velocity measurements at the bisector levels 0.5, 0.75, and line core, respectively. The axes of these first three panels represent the wave numbers k_x and k_y. The cross amplitude power in these 2D spectra is indicated by different colors from red (maximum power) to green (minimum power). The insets show the corresponding mean azimuthal cross-amplitude power Q_a(k_r) . The lower right panel shows the three mean azimuthal cross amplitude spectra  (full line), (dashed), and  (long dashed), respectively, normalized to the mean azimuthal velocity power at the line bisector 0.2, .