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| Titel |
Polar cap ion beams during periods of northward IMF: Cluster statistical results |
| VerfasserIn |
R. Maggiolo, M. Echim, J. Keyser, D. Fontaine, C. Jacquey, I. Dandouras |
| 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. 5 ; Nr. 29, no. 5 (2011-05-06), S.771-787 |
| Datensatznummer |
250017012
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| Publikation (Nr.) |
 copernicus.org/angeo-29-771-2011.pdf |
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| Zusammenfassung |
Above the polar caps and during prolonged periods of northward IMF, the
Cluster satellites detect upward accelerated ion beams with energies up to a
few keV. They are associated with converging electric field structures
indicating that the acceleration is caused by a quasi-static field-aligned
electric field that can extend to altitudes higher than 7 RE (Maggiolo
et al., 2006; Teste et al., 2007).
Using the AMDA science analysis service provided by the Centre de
Données de la Physique des Plasmas, we have been able to extract about
200 events of accelerated upgoing ion beams above the polar caps from the
Cluster database. Most of these observations are taken at altitudes lower
than 7 RE and in the Northern Hemisphere.
We investigate the statistical properties of these ion beams. We analyze
their geometry, the properties of the plasma populations and of the electric
field inside and around the beams, as well as their dependence on solar wind
and IMF conditions. We show that ~40 % of the ion beams are
collocated with a relatively hot and isotropic plasma population. The
density and temperature of the isotropic population are highly variable but
suggest that this plasma originates from the plasma sheet. The ion beam
properties do not change significantly when the isotropic, hot background
population is present. Furthermore, during one single polar cap crossing by
Cluster it is possible to detect upgoing ion beams both with and without an
accompanying isotropic component.
The analysis of the variation of the IMF BZ component prior to the
detection of the beams indicates that the delay between a
northward/southward turning of IMF and the appearance/disappearance of the
beams is respectively ~2 h and 20 min. The observed
electrodynamic characteristics of high altitude polar cap ion beams suggest
that they are closely connected to polar cap auroral arcs. We discuss the
implications of these Cluster observations above the polar cap on the
magnetospheric dynamics and configuration during prolonged periods of
northward IMF. |
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