Upper panel: Hilbert spectrum (the instant frequency-time distribution) of empirically determined intrinsic modes 3, 4, 5. Darker pixels correspond to higher instant spectral power, bottom panel: Fourier power spectrum of the signal obtained as the sum of intrinsic modes 3, 4, 5. The Fourier power is normalised to the highest value. The elapsed time on the horizontal axis starts at 01:05:22 UT. The narrow-band Gaussian filters at 0.005–0.015 Hz, 0.015–0.03 Hz, 0.04–0.08 Hz are shown by the dotted curves.