Radio spectrograms (frequency versus time) between 10 and 160 kHz of SORBET spectrometer obtained during Mercury’s flyby on 1October 2021 (MSB1 on top), and 23 June 2022 (MSB2 on bottom), respectively (the cadence was one spectrum every 4 s), with a color bar chart on the right indicating the level (in dB). Superimposed upper and lower white curves (dots) show the plasma frequency limits, f_p, sup and f_p, inf, respectively, as determined by the QTN plateau detection. The red dots shows the f_p, proxy as explained in the text, and, on top panel only, the smoothed black curve indicates the frequency of the spectral absolute minimum strictly below f_p, sup.