Autocovariance functions of intensity time series. (A) Autocovariance function of the VIRGO/SPM data (T = 90 d). The red, green, and blue curves correspond to the three channels. (B) Autocovariance of MURaM solar convection simulations (T = 15 h). (C) Autocovariance function for the p-mode data computed for a segment of VIRGO data with T = 90 days (red channel). (D) Reference autocovariance function obtained by averaging over M = 91 consecutive segments covering 22 yr. The first seven wavepackets are labeled s = 1 through 7. The curves in panels C and D are computed from filtered time series (Gaussian filter centered on 3 mHz with a full width of 2 mHz). (E) Zoom onto wavepacket s = 4 and width of the temporal window function f₄(t).