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Ann. Geophys., 19, 1241-1258, 2001
www.ann-geophys.net/19/1241/2001/
© European Geosciences Union 2001
Early results from the Whisper instrument on Cluster: an overview
P. M. E. Décréau1, P. Fergeau1, V. Krasnoselskikh1, E. Le Guirriec1, M. Lévêque1, Ph. Martin1, O. Randriamboarison1, J. L. Rauch1, F. X. Sené1, H. C. Séran1, J. G. Trotignon1, P. Canu2, N. Cornilleau2, H. de Féraudy2, H. Alleyne3, K. Yearby3, P. B. Mögensen4, G. Gustafsson5, M. André5, D. C. Gurnett6, F. Darrouzet7, J. Lemaire7, C. C. Harvey8, P. Travnicek9, and Whisper experimenters (Table 1)*
1LPCE/CNRS and Université d’Orléans, Orléans, France
2CETP/CNRS and VSQP University, Vélizy, France
3University of Sheffield, Sheffield, UK
4DSRI, Copenhaguen, Denmark
5I. R. F. U., Uppsala, Sweden
6University of Iowa, Iowa, USA
7Institut d’Aéronomie Spatiale de Belgique, Bruxelles, Belgium
8CESR, Toulouse, France
9Czech Academy of Science, Prague, Czech Republic
*The Whisper team deeply regrets the untimely demise of Les Woolliscroft, PI of the DWP instrument and Co-I of Whisper. He played a key role in the Whisper instrument’s capabilities.
Abstract. The Whisper instrument
yields two data sets: (i) the electron density determined via the relaxation
sounder, and (ii) the spectrum of natural plasma emissions in the frequency
band 2–80 kHz. Both data sets allow for the three-dimensional exploration of
the magnetosphere by the Cluster mission. The total electron density can be
derived unambiguously by the sounder in most magnetospheric regions, provided
it is in the range of 0.25 to 80 cm-3 . The natural emissions
already observed by earlier spacecraft are fairly well measured by the Whisper
instrument, thanks to the digital technology which largely overcomes the
limited telemetry allocation. The natural emissions are usually related to the
plasma frequency, as identified by the sounder, and the combination of an
active sounding operation and a passive survey operation provides a time
resolution for the total density determination of 2.2 s in normal telemetry
mode and 0.3 s in burst mode telemetry, respectively. Recorded on board the
four spacecraft, the Whisper density data set forms a reference for other
techniques measuring the electron population. We give examples of Whisper
density data used to derive the vector gradient, and estimate the drift
velocity of density structures. Wave observations are also of crucial interest
for studying small-scale structures, as demonstrated in an example in the
fore-shock region. Early results from the Whisper instrument are very
encouraging, and demonstrate that the four-point Cluster measurements indeed
bring a unique and completely novel view of the regions explored.
Key words. Space plasma physics
(instruments and techniques; discontinuities, general or miscellaneous)
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