Top panel: two-dimensional FFT power spectrum of the time-distance map shown in the bottom panel of Fig. 1 as a function of spatial frequency, k, and temporal frequency, ν. The power is normalised with respect to the variance of the time series signal within each pixel. The peak power occurs at a fundamental frequency of ν₁ ≈ 1.33 mHz, corresponding to the dominant 13 min period of the loop. Signatures of high-frequency components are also present at ν > ν₁ with decreasing power. Bottom panel: log-log plot of the FFT power as a function of frequency. The black dots represent the distribution of spectral power within each pixel domain. The solid black line indicates the mean power and the dashed line indicates the 95% confidence level estimated from the theoretical white-noise spectrum. The shaded red region highlights a secondary significant frequency band with a peak at ∼2.67 mHz.