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| Titel |
Wide-banded NTC radiation: local to remote observations by the four Cluster satellites |
| VerfasserIn |
P. M. E. Décréau, S. Aoutou, A. Denazelle, I. Galkina, J.-L. Rauch, 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 ; 33, no. 10 ; Nr. 33, no. 10 (2015-10-22), S.1285-1300 |
| Datensatznummer |
250121259
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| Publikation (Nr.) |
 copernicus.org/angeo-33-1285-2015.pdf |
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| Zusammenfassung |
| The Cluster multi-point mission offers a unique collection of non-thermal
continuum (NTC) radio waves observed in the 2–80 kHz frequency range over
almost 15 years, from various view points over the radiating plasmasphere.
Here we present rather infrequent case events, such as when primary electrostatic
sources of such waves are embedded within the plasmapause boundary far from
the magnetic equatorial plane. The spectral signature of the emitted
electromagnetic waves is structured as a series of wide harmonic bands
within the range covered by the step in plasma frequency encountered at the
boundary. Developing the concept that the frequency distance df between
harmonic bands measures the magnetic field magnitude B at
the source (df = Fce, electron gyrofrequency), we analyse three selected
events. The first one (studied in Grimald et al., 2008) presents electric
field signatures observed by a Cluster constellation of small size
(~ 200 to 1000 km spacecraft separation) placed in the
vicinity of sources. The electric field frequency spectra display frequency
peaks placed at frequencies fs = n df (n being an integer), with df of the order of
Fce values encountered at the plasmapause by the spacecraft. The second
event, taken from the Cluster tilt campaign, leads to a 3-D view of NTC waves
ray path orientations and to a localization of a global source region at
several Earth radii (RE) from Cluster (Décréau et al., 2013).
The measured spectra present successive peaks placed at fs ~ (n+ 1/2) df. Next, considering if both situations might be two facets of the
same phenomenon, we analyze a third event. The Cluster fleet, configured
into a constellation of large size (~ 8000 to 25 000 km spacecraft separation),
allows us to observe wide-banded NTC waves at
different distances from their sources. Two new findings can be derived from
our analysis. First, we point out that a large portion of the plasmasphere
boundary layer, covering a large range of magnetic latitudes, is radiating
radio waves. The radio waves are issued from multiple sources of small size,
each related to a given fs series and radiating inside a beam of narrow cone
angle, referred to as a beamlet. The beamlets illuminate different
satellites simultaneously, at different characteristic fs values, according to
the latitude at which the satellite is placed. Second, when an observing
satellite moves away from its assumed source region (the plasmapause
surface), it is illuminated by several beamlets, issued from nearby sources
with characteristic fs values close to each other. The addition of radio waves
blurs the spectra of the overall received electric field. It can move the
signal peaks such that their position fs satisfiesfs = (n+α) df, with
0 < α < 1. These findings open new perspectives for
the interpretation of NTC events displaying harmonic signatures. |
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