The Cluster mission
It consists of 4 identical satellites, launched in two pairs in August and December 2000, using Soyuz rockets from the Baikonur site (Kazakhstan). The scientific objectives of the mission are focused on the study of the interactions of the Solar Wind and the Earth’s magnetosphere and its different regions and boundaries. These could be explored in detail thanks to an orbit evolving during the mission with a perigee of 400 km and an apogee of 12 Earth radii (80,000 km).
Artist’s view of the 4 CLUSTER satellites – Credit ESA.
Each satellite, with a mass of one ton, carries eleven instruments dedicated to the measurement of DC and AC electric and magnetic fields, as well as electron and ion populations.
Satellite instrumentation and associated measurements: ions, electrons (2, 5, 6), magnetic field (4), electric field (3,8), electromagnetic waves (7, 9, 10, 11), satellite potential control (1). Photo credit ESA
The CLUSTER-WHISPER instrument
WHISPER (Waves of HIgh frequency and Sounder for Probing of Electron density by Relaxation) is one of the eleven instruments on board each of the four satellites Cluster.
The WHISPER instrument is dedicated to two fundamental functions:
A first function consists in measuring the fluctuations of the high frequency electric field in the [2-80 kHz] band. These electric field variations can be of local origin (electrostatic waves) or from distant sources (electromagnetic waves).
The second function is the measurement of the electron density of the surrounding plasma. The technique used is that of the relaxation sounder which consists in exciting the local resonances of the plasma by emitting a powerful electric pulse at very precise frequencies. The analysis of the spectrum of the response of the medium and the identification of the resonances allows to deduce with an excellent precision the electronic density, key parameter of the plasma. This measurement gives access to the local gradients of the density and to the spatial distribution of electrons in the different regions of the terrestrial magnetosphere. Moreover, this measurement is used for the recalibration of particle instruments and Langmuir probes.
Example of plasma signatures, measured by the WHISPER instrument, in the different regions of the magnetosphere.
The design and the technical realization of WHISPER were entirely carried out at LPC2E. The instrument consists of an electronic box, coupled to the electrical antennas (80m long) of the satellite. It manages the transmission, reception and on-board analysis of the signal in real time. After more than 20 years of mission, the four WHISPER instruments continue to operate nominally and the data, distributed to the international scientific community through the ESA archive center, are still actively exploited.
Contact at LPC2E : Pierre.Henri@cnrs-orleans.fr
