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| Titel |
Remote sensing of a NTC radio source from a Cluster tilted spacecraft pair |
| VerfasserIn |
P. M. E. Décréau, S. Kougblénou, G. Lointier, J.-L. Rauch, J.-G. Trotignon, X. Vallières, P. Canu, S. Rochel Grimald, F. El-Lemdani Mazouz, F. Darrouzet |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 31, no. 11 ; Nr. 31, no. 11 (2013-11-26), S.2097-2121 |
| Datensatznummer |
250086154
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| Publikation (Nr.) |
 copernicus.org/angeo-31-2097-2013.pdf |
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| Zusammenfassung |
| The Cluster mission operated a "tilt campaign" during the month of May 2008.
Two of the four identical Cluster spacecraft were placed at a
close distance (~50 km) from each other and the spin axis of
one of the spacecraft pair was tilted by an angle of
~46°. This gave the opportunity, for the first time in space, to
measure global characteristics of AC electric field, at the sensitivity
available with long boom (88 m) antennas, simultaneously from the specific
configuration of the tilted pair of satellites and from the available base
of three satellites placed at a large characteristic separation
(~1 RE). This paper describes how global characteristics
of radio waves, in this case the configuration of the electric field
polarization ellipse in 3-D-space, are identified from in situ
measurements of spin modulation features by the tilted pair, validating a
novel experimental concept. In the event selected for analysis, non-thermal
continuum (NTC) waves in the 15–25 kHz frequency range are observed from
the Cluster constellation placed above the polar cap. The observed intensity
variations with spin angle are those of plane waves, with an electric field
polarization close to circular, at an ellipticity ratio e = 0.87. We
derive the source position in 3-D by two different methods. The first one uses ray path orientation (measured by the tilted
pair) combined with spectral signature of magnetic field magnitude at
source. The second one is obtained via triangulation from the three
spacecraft baseline, using estimation of directivity angles under assumption
of circular polarization. The two results are not compatible, placing
sources widely apart. We present a general study of the level of systematic
errors due to the assumption of circular polarization, linked to the second
approach, and show how this approach can lead to poor triangulation and
wrong source positioning. The estimation derived from the first method
places the NTC source region in the dawn sector, at a large L value (L
~ 10) and a medium geomagnetic latitude (35° S).
We discuss these untypical results within the frame of the geophysical
conditions prevailing that day, i.e. a particularly
quiet long time interval, followed by a short increase of magnetic activity. |
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