The WHISPER (Waves of High frequency and Sounder for Probing of Electron density by Relaxation) aboard the four CLUSTER satellites performs high frequency electric field measurements in the Earth's space environment, from the solar wind down to the plasmasphere. In active mode, the WHISPER behaves like a topside sounder. It provides, via the identification of resonances that are triggered at characteristic frequencies of the encountered ionised medium, reliable and accurate determination of the total density of thermal electrons. In passive mode, when the transmitter is switched off, it becomes a classical wave receiver. The 2 to 80 kHz frequency range of the electric component of natural waves is therefore monitored. The objective of the presentation is to show that the thermal electron density may be derived from the active and/or passive measurements of the WHISPER all along the CLUSTER trajectory, and almost everywhere, with a good accuracy compared with other estimates.