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Ann. Geophys., 22, 2607-2624, 2004
www.ann-geophys.net/22/2607/2004/
© European Geosciences Union 2004
Observation of continuum radiations from the Cluster fleet: first results from direction finding
P. M. E. Décréau1, C. Ducoin1,*, G. Le Rouzic1, O. Randriamboarison1, J.-L. Rauch1, J.-G. Trotignon1, X. Vallières1, P. Canu2, F. Darrouzet3, M. P. Gough4, A. M. Buckley4, and T. D. Carozzi4
1LPCE, CNRS et Université d’Orléans, Orléans, France
2CETP, CNRS et Université Versailles St Quentin, Vélizy, France
3Belgian Institute for Space Aeronomy, (IASB-BIRA), Brussels, Belgium
4Space Science Centre, the University of Sussex, Falmer, Brighton, UK
*March-June 2003 visit at LPCE
Abstract. The Cluster fleet offers the first possibility of comparing
non-thermal terrestrial continuum radiation from similarly equipped nearby
observation points. A very rich data set has already been acquired on the
Cluster polar orbit, between 4 and 19 Earth radii geocentric distances,
and preliminary analysis has been carried out on these emissions. We focus
in this paper on direction finding performed from all four spacecraft as
a means to locate the position of the sources of this continuum radiation.
Directions are derived from spin modulation properties, under the usual
analysis assumptions of the wave vector of the radiation lying in the plane
containing the spin axis and the antenna position at electric field minimum.
All the spin axes of the four Cluster spacecraft are aligned perpendicular
to the ecliptic, thus the aligned spacecraft antenna spin planes provide
redundant 2-D views of the propagation path of the radiation and source location.
Convincing 2-D triangulation results have been obtained in the vicinity of
the source region. In addition, the out of spin plane component of the wave
vector reveals itself to a certain extent through directivity
characteristics compared at different distances of the spin plane to the
ecliptic. The four case events studied (two of them taken near apogee, the
other two near perigee) have confirmed general properties derived from previous
observations: trapping in the lower frequency range, radiation escaping into
the magnetosheath region in the higher frequency range. All propagation
directions are compatible with source positions in the plasmapause region,
however, at a significant distance from the equator in one case. Our
observations have also revealed new properties, like the importance of
small-scale density irregularities in the local amplification of continuum
radiation. We conclude that more detailed generation and propagation models
are needed to fit the observations.
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