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Ann. Geophys., 25, 823-831, 2007
www.ann-geophys.net/25/823/2007/
© European Geosciences Union 2007
A quantitative test of Jones NTC beaming theory using CLUSTER constellation
S. Grimald1, P. M. E. Décréau1, P. Canu2, X. Suraud1, X. Vallières1, F. Darrouzet3, and C. C. Harvey4
1Laboratoire de Physique et Chimie de l'Environnement (LPCE), Orléans, France
2Centre d'Etude des Environnements Terrestre et Planétaires (CETP), Vélizy, France
3Belgian Institute for Space Aeronomy (IASB-BIRA), Brussels, Belgium
4Centre d'Etude Spatiale des Rayonnements (CESR), Toulouse, France
Abstract. Non-thermal continuum (NTC) radiation is, with auroral kilometric radiation
(AKR), one of the two electromagnetic emissions generated within the Earth's
magnetosphere and radiated into space. The location of the source of NTC has
been sought for several decades, with only limited success. The
constellation formed by the four CLUSTER spacecraft provides the possibility
of triangulation in the vicinity of the source, thus allowing progress in
source localisation, while simultaneously revealing the beaming properties
of NTC radio sources.
We present a case event showing two beams localised on opposite sides of the
magnetic equator. At any selected frequency, triangulation points to a
single region source of small size. Its position is compatible with the
range of possible loci of sources predicted by the radio window theory of
Jones (1982) in a frame of constraints relaxed from the simple sketch
proposed in early works. The analysis of similar observations from the
Dynamics Explorer 1 by Jones et al. (1987) enabled the authors to claim
validation of the radio window theory. CLUSTER observations, however, reveal
a large beaming cone angle projected onto the ecliptic plane, a feature
unobservable by Dynamics Explorer which had a different spin axis
orientation. According to the radio window theory, such a large observed
cone angle can only be formed by a series of point sources, each beaming in
a narrow cone angle. This study demonstrates the difficulty of validating
NTC linear generation mechanisms using global beaming properties alone.
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