Upper panel: TNR dynamic spectrum of the V1 − V2 (PZ-PY) dipole signal. The plasma frequency can clearly be seen and varies between roughly 55 and 75 kHz. Using a peak tracking technique it is possible to detect the plasma frequency and deduce the temporal variation of the local electron density N_e − fp displayed in red on the lower panel. Superimposed in black on the lower panel is the electron density N_{e − S/C} deduced from the spacecraft potential (Khotyaintsev et al. 2021), after calibration to N_e − fp over longer time interval.