Time evolution of individual horizontal mode Fourier harmonics, computed from the vertical velocity of neutrals, as a function of time. The colored lines show the Fourier amplitudes of the harmonics, grouped in sets of four, from n = 1 to n = 48 (see legend at right). The amplitudes are averaged between heights −L₀ and L₀ and within each group. The results for four simulations are shown: P (top left); P-V0 (top right); L2 (bottom left); L2-α (bottom right).