Time-frequency diagram of the A_PEN time series. The top panel displays the observed data along with the red curve providing the fit by a polynomial to the long-term variations based on a degree of 6. The color-scale image provides the evolution of the Fourier power spectrum with the epoch on the x-axis. Violet, dark blue, light blue, and cyan colors stand for power ≥8 × 10⁻⁵, ≥4 × 10⁻⁵, ≥2 × 10⁻⁵, and ≥10⁻⁵. The right vertical panel illustrates the Fourier power spectrum evaluated over the full duration of the USET time series. The colored straight lines correspond to the scale used in the time-frequency diagram.