Evolution of the peak’s amplitude as a function of time. High-frequency waves (violet) have a lifetime much greater than 550 days since their amplitude does not vary over this time interval. Intermediate-frequency waves (red) are damped along their propagation and for low-frequency waves (black) their amplitude sometimes increases suddenly, due to a re-excitation. The legend indicates the values of (ℓ, n) and the corresponding frequency f in mHz for each curve.