Impact of the WGN on the periodograms. Top: comparison of the periodogram of HD 67458 (gray) and the averaged periodograms of three solar 8-h subseries (green). Best-fitting models resulting from Harvey-functions (Eq. (1)) fits on these CHEOPS and VIRGO periodograms are shown in yellow and green, respectively. Bottom: same comparison but with the averaged periodogram of three solar subseries with the WGN added (pink). The associated best-fitting Harvey function is shown in red. We see how the WGN impacts the periodogram shape (and so the extraction of the granulation properties) by comparing the green and red solid lines. In both panels, the different cut-off frequencies ν_cut, f_H, and f_g inferred on CHEOPS observations are represented by the green, cyan and magenta vertical lines, respectively.