Observed f_p line (in black, linear scale) superimposed on the supply interference line (in red) scaled in amplitude and shifted in frequency. The f_p line profile was obtained by averaging 45 300 good-quality spectra which were shifted to a common central frequency, fitted to a Gaussian profile, and acquired within the period shown in Fig. 1. The total power exceeds the background by about 20%, which means that the contribution of the line amounts to roughly 20% of the background.