WHISPER, A RESONANCE SOUNDER AND WAVE ANALYZER - PERFORMANCES AND PERSPECTIVES FOR THE CLUSTER MISSION

The WHISPER sounder on the Cluster spacecraft is primarily designed to provide an absolute measurement of the total plasma density within th e range 0.2-80 cm(-3). This is achieved by means of a resonance soundi ng technique which has already proved successful in the regions to be explored. The wave analysis function of the instrument is provided by FFT calculation. Compared with the swept frequency wave analysis of pr evious sounders, this technique has several new capabilities. In parti cular, when used for natural wave measurements (which cover here the 2 -80 kHz range), it offers a flexible trade-off between time and freque ncy resolutions. In the basic nominal operational mode, the density is measured every 28 s, the frequency and time resolution for the wave m easurements are about 600 Hz and 2.2 s, respectively. Better resolutio ns can be obtained, especially when the spacecraft telemetry is in bur st mode. Special attention has been paid to the coordination of WHISPE R operations with the wave instruments, as well as with the low-energy particle counters. When operated from the multi-spacecraft Cluster, t he WHISPER instrument is expected to contribute in particular to the s tudy of plasma waves in the electron foreshock and solar wind, to inve stigations about small-scale structures via density and high-frequency emission signatures, and to the analysis of the non-thermal continuum in the magnetosphere.