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| Titel |
Polar cap arcs from the magnetosphere to the ionosphere: kinetic modelling and observations by Cluster and TIMED |
| VerfasserIn |
R. Maggiolo, M. Echim, C. Simon Wedlund, Y. Zhang, D. Fontaine, G. Lointier, J.-G. Trotignon |
| 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 ; 30, no. 2 ; Nr. 30, no. 2 (2012-02-03), S.283-302 |
| Datensatznummer |
250017179
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| Publikation (Nr.) |
 copernicus.org/angeo-30-283-2012.pdf |
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| Zusammenfassung |
On 1 April 2004 the GUVI imager onboard the TIMED spacecraft spots an
isolated and elongated polar cap arc. About 20 min later, the Cluster
satellites detect an isolated upflowing ion beam above the polar cap.
Cluster observations show that the ions are accelerated upward by a
quasi-stationary electric field. The field-aligned potential drop is
estimated to about 700 V and the upflowing ions are accompanied by a tenuous
population of isotropic protons with a temperature of about 500 eV.
The magnetic footpoints of the ion outflows observed by Cluster are situated
in the prolongation of the polar cap arc observed by TIMED GUVI. The
upflowing ion beam and the polar cap arc may be different signatures of the
same phenomenon, as suggested by a recent statistical study of polar cap ion
beams using Cluster data.
We use Cluster observations at high altitude as input to a quasi-stationary
magnetosphere-ionosphere (MI) coupling model. Using a Knight-type
current-voltage relationship and the current continuity at the topside
ionosphere, the model computes the energy spectrum of precipitating
electrons at the top of the ionosphere corresponding to the generator
electric field observed by Cluster. The MI coupling model provides a
field-aligned potential drop in agreement with Cluster observations of
upflowing ions and a spatial scale of the polar cap arc consistent with the
optical observations by TIMED. The computed energy spectrum of the
precipitating electrons is used as input to the Trans4 ionospheric transport
code. This 1-D model, based on Boltzmann's kinetic formalism, takes into
account ionospheric processes such as photoionization and electron/proton
precipitation, and computes the optical and UV emissions due to
precipitating electrons. The emission rates provided by the Trans4 code are
compared to the optical observations by TIMED. They are similar in size and
intensity. Data and modelling results are consistent with the scenario of
quasi-static acceleration of electrons that generate a polar cap arc as they
precipitate in the ionosphere. The detailed observations of the acceleration
region by Cluster and the large scale image of the polar cap arc provided by
TIMED are two different features of the same phenomenon. Combined together,
they bring new light on the configuration of the high-latitude magnetosphere
during prolonged periods of Northward IMF. Possible implications of the
modelling results for optical observations of polar cap arcs are also
discussed. |
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