Intensity given by I(x, y) (see text) mimicking a GONG Hα map. The dark area corresponds to the filament seen in absorption. The filament oscillates with a period, P = 60 min, damping time, τ = 75 min, and a direction of 45° with respect to the x-axis. The dotted isocontours at x >  0 show the position of the filament at t = P/4 when the filament reaches its maximum elongation right of the equilibrium position. Similarly, the dotted isocontours at x <  0 show the filament at t = 3/4P when the filament reaches the maximum elongation at the left. The six red circles show the positions along the oscillation trajectory where the intensity fluctuations are studied in detail in Fig. 2.