Left panel: autocorrelation of the power spectrum as obtained by Fossat et al. (2017) as a function of frequency lag for the GONG instrument (top) and the BiSON instrument (bottom). The green vertical lines correspond to frequencies at 210 nHz, 630 nHz and 1260 nHz. The black line indicates the mean value, while the orange line indicates the 10% probability level that there is at least one peak due to noise in the window; the level is 3.46σ for GONG, and 3.48σ for BiSON. Right panel: sum of the correlation for l = 1, and l = 2 modes as obtained by Fossat et al. (2017) as a function of rotation frequency for the GONG instrument (top) and the BiSON instrument (bottom). The black line indicates the mean value, while the orange line indicates the 10% probability level that there is at least one peak due to noise in the window; the level is 3.43σ.