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| Titel |
Transients in oxygen outflow above the polar cap as observed by the Cluster spacecraft |
| VerfasserIn |
H. Nilsson, M. Waara, O. Marghitu, M. Yamauchi, R. Lundin, H. Rème, J.-A. Sauvaud, I. Dandouras, E. Lucek, L. M. Kistler, B. Klecker, C. W. Carlson, M. B. Bavassano-Cattaneo, A. Korth |
| 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 ; 26, no. 11 ; Nr. 26, no. 11 (2008-10-21), S.3365-3373 |
| Datensatznummer |
250016277
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| Publikation (Nr.) |
 copernicus.org/angeo-26-3365-2008.pdf |
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| Zusammenfassung |
| Oxygen ion outflow associated with the cusp and cleft give rise to persistent
oxygen ion beams which can be observed over the polar cap. For high altitude
spacecraft such as Cluster these beams are often observed for several hours
on each occasion. This allows for a study of typical temporal structures on
the time scale of minutes. We have used 3 years of data from spring, January
to May of years 2001 to 2003, for a study of the oxygen number flux variation
in the polar cap ion outflow. The source of these oxygen ion beams is the
cusp and cleft, and variations in ionospheric upflow on time scales of around
8 min have been reported from ground based studies using incoherent scatter
radar. Such upflows typically do not reach escape velocity, and further
energization above the ionosphere is required for outflow to occur. Our study
shows that a typical time scale between sudden number flux enhancements
observed by Cluster in a geocentric distance range of 5 RE to 12 RE is 5
to 10 min. A superposed epoch study does not reveal any
significant convection velocity or temperature changes around the flux
enhancement events. Sudden temperature enhancements occur with a typical time
interval of about 4 min, A superposed epoch study does not reveal any
number flux enhancements associated with the temperature enhancements. The
clear modulation of the high altitude number flux in a manner which resembles
the modulation of the ionospheric upflow indicates that this is the main
limiting factor determining the total outflow. The process behind transient
upflow events in the ionosphere is therefore important for the total
ionospheric outflow. Subsequent heating above the ionosphere appears to be
common enough in the cusp/cleft region that it does not significantly
modulate the oxygen ion number flux. |
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