Short-time Fourier transformation of the frequency region between 1.5 and 2.3 d^-1 with a rectangular window of width 30 d^-1. All signal outside of these regions is removed. Clear signal is present (colours denote squared amplitude in ppm²), and the frequencies and amplitudes show hints of variability. As a reference, the top panel shows a copy of the upper part of plot, where two artificial constant sine curves are introduced.