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| Titel |
Lower hybrid resonances stimulated by the four CLUSTER relaxation sounders deep inside the plasmasphere: observations and inferred plasma characteristics |
| VerfasserIn |
S. Kougblénou, G. Lointier, P. M. E. Décréau, J.-G. Trotignon, J.-L. Rauch, X. Vallières, P. Canu, A. Masson, J. Pickett |
| 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 ; 29, no. 11 ; Nr. 29, no. 11 (2011-11-09), S.2003-2018 |
| Datensatznummer |
250017123
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| Publikation (Nr.) |
 copernicus.org/angeo-29-2003-2011.pdf |
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| Zusammenfassung |
| The frequency range of the WHISPER relaxation sounder instrument on board
CLUSTER, 4–80 kHz, has been chosen so as to encompass the electron
gyro-frequency, Fce, and the electron plasma frequency, Fp, in
most regions to be explored. Measurement of those frequencies, which are
triggered as resonances by the sounder, provides a direct estimation of in
situ fundamental plasma characteristics: electron density and magnetic field
intensity. In the late mission phase, CLUSTER penetrated regions deep inside
the plasmasphere where Fce and Fp are much higher than the upper
frequency of the sounder's range. However, they are of the right order of
magnitude as to place the lower hybrid frequency, Flh, in the 4–15 kHz
band. This characteristic frequency, placed at a resonance of the
medium, is triggered by the sounder's transmitter and shows up as an
isolated peak in the received spectrum, not present in spectra of naturally
occuring VLF waves. This paper illustrates, from analysis of case events,
how measured Flh values give access to a plasma diagnostic novel of its
kind. CLUSTER, travelling along its orbit, encounters favourable conditions
where Fce is increasing and Fp decreasing, such that
Fce/Fp increases from values below unity to values above unity.
Measured Flh values thus give access, in turn, to the effective mass,
Meff, indicative of plasma ion composition, and to the core
plasmasphere electron density value, a parameter difficult to measure. The
analysed case events indicate that the estimated quantities (Meff in
the 1.0–1.4 range, Ne in the 5 × 102–104 cm−3 range)
are varying with external factors (altitude, L value, geomagnetic activity)
in a plausible way. Although covering only a restricted region (mid-latitude, low altitude inner plasmasphere), these measurements are
available, since late 2009, for all CLUSTER perigee passes not affected by
eclipses (on average, roughly a third of a total of ~200 passes per
year) and offer multipoint observations previously unavailable in this
region. |
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