WHISPER (Waves of HIgh frequency and Sounder for Probing the Electron density by Relaxation) is a component of the Wave Experiment Consortium (WEC). The instrumentation consists of a resonance sounder, which has two modes of operation, active and passive, which provide respectively:

• the total electron density using a relaxation sounder in the range 0.1-80 e cm^-3
• a survey of the natural wave electric field in the frequency range 2-80 kHz.

The resonance, or relaxation, sounder is an active wave experiment, first flown in the terrestrial magnetosphere on board the GEOS 1-2, ISEE 1, and Viking satellites, as well as on Ulysses in the solar wind and Jovian magnetosphere. The principle of the instrument is similar to that of a radar: an emitter sends a train of pulses in the plasma over a frequency range which contains the electron plasma frequency. When the pulse frequency matches one of the characteristic frequencies of the plasma Fp the plasma frequency, Fuh the upper hybrid frequency, nFce, the harmonics of the electron gyrofrequency, and the so-called Fq which are the frequencies of the zero-group velocity of the Bernstein modes) a strong resonance is observed by the receiver. The processing of the set of observed resonances in a sounder spectrum determines the plasma frequency from which the total electron density Ne is derived.
Although the WHISPER sounder has evolved from similar instruments flown on past satellites, the specific constraints of the Cluster project required considering a different technology. The transmission of pulses is still organised by stepping the frequency as on earlier relaxation sounders, but the WHISPER receiver analyses the signal by FFT over the full frequency range, whereas previous instruments used a swept frequency analyser.
In sounding modes, the frequency range of the emitter is explored with a maximum of 83 steps each of bandwidth 980 Hz. The signal is received through the 100 m tip-to-tip sphere antennas of the EFW experiment and analysed by a 512-bin FFT, over the range 0-81 kHz. This provides a bin separation of 162 Hz, with a dynamic range of 66 dB. An active wave spectrum is reconstructed by selection, at each step, of the frequency bins in the 980 Hz bandwidth covered by the central frequency of the pulse. It is compressed and sent to the telemetry by the WEC-DWP. The software can also process the natural signal detected in the previous spectrum a few tens of a millisecond before the actual stimulation, and reconstructs the natural wave spectrum.
In natural wave modes, the emitter is off, and the waves are studied with different times and frequency resolutions in the range 2-80 kHz. Four FFT dimensions are available (512, 256, 128, 64 bins with a frequency resolution of 162, 324, 648 and 1296 Hz respectively). Different WHISPER sounding and natural waves modes as well as DWP data selection and compression schemes are implemented to cope with the available resources on board, scientific interests, and WEC modes of operations.

This description is derived from Section 3.11 of the ''Users Guide to the Cluster Science Data System'', DS-MPA-TN-0015.