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| Titel |
Naturally enhanced ion-line spectra around the equatorial 150-km region |
| VerfasserIn |
J. L. Chau, R. F. Woodman, M. A. Milla, E. Kudeki |
| 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 ; 27, no. 3 ; Nr. 27, no. 3 (2009-03-02), S.933-942 |
| Datensatznummer |
250016428
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| Publikation (Nr.) |
 copernicus.org/angeo-27-933-2009.pdf |
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| Zusammenfassung |
| For many years strong radar echoes coming from 140–170 km
altitudes at low latitudes have been associated to the existence of
field-aligned irregularities (FAIs) (the so called 150-km echoes). In this
work, we present frequency spectra as well as angular distribution of 150-km
echoes. When the 150-km region is observed with beams perpendicular to the
magnetic field (B) the observed radar spectra are very narrow with
spectral widths between 3–12 m/s. On the other hand, when few-degrees
off-perpendicular beams are used, the radar spectra are wide with spectral
widths comparable to those expected from ion-acoustic waves at these
altitudes (>1000 m/s). Moreover the off-perpendicular spectral width
increases with increasing altitude. The strength of the received echoes is
one to two orders of magnitude stronger than the expected level of waves in
thermal equilibrium at these altitudes. Such enhancement is not due to an
increase in electron density. Except for the enhancement in power, the
spectra characteristics of off-perpendicular and perpendicular echoes are in
reasonable agreement with expected incoherent scatter spectra at these angles
and altitudes. 150-km echoes are usually observed in narrow layers (2 to 5).
Bistatic common volume observations as well as observations made few
kilometers apart show that, for most of the layers, there is very high
correlation on power fluctuations without a noticeable time separation
between simultaneous echoes observed with Off-perpendicular and Perpendicular
beams. However, in one of the central layers, the echoes are the strongest in
the perpendicular beam and absent or very weak in the off-perpendicular
beams, suggesting that they are generated by a plasma instability. Our
results indicate that most echoes around 150-km region are not as aspect
sensitive as originally thought, and they come from waves that have been
enhanced above waves in thermal equilibrium. |
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