(a) Figure 11a from the main article file and (b) Figure 13 from the main article file (with the horizontal axis cropped). The arrowed positions, numbered 1 – 4, show periodic structures in the HF Doppler spectrogram that match the crests of the waveform in the bottom figure. Apart from the tilt of the numbered structures in the HF Doppler spectrogram, the periodic structures in the upper plot and the crests of the waveform in the lower plot show a one-to-one correspondence.