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| Titel |
Characteristics of quasi-static potential structures observed in the auroral return current region by Cluster |
| VerfasserIn |
G. T. Marklund, T. Karlsson, S. Figueiredo, T. Johansson, P.-A. Lindqvist, M. André, S. Buchert, L. M. Kistler, A. Fazakerley |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1023-5809
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| Digitales Dokument |
URL |
| Erschienen |
In: Nonlinear Processes in Geophysics ; 11, no. 5/6 ; Nr. 11, no. 5/6 (2004-12-21), S.709-720 |
| Datensatznummer |
250008999
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| Publikation (Nr.) |
 copernicus.org/npg-11-709-2004.pdf |
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| Zusammenfassung |
| Temporal and spatial characteristics of intense quasi-static electric fields
and associated electric potential structures in the return current region
are discussed using Cluster observations at geocentric distances of about 5
Earth radii. Results are presented from four Cluster encounters with such
acceleration structures to illustrate common as well as different features
of such structures. The electric field structures are characterized by (all
values are projected to 100 km altitude) peak amplitudes of ≈1V/m,
bipolar or unipolar profiles, perpendicular scale sizes of ≈10km,
occurrence at auroral plasma boundaries associated with plasma
density gradients, downward field-aligned currents of ≈10µA/m2,
and upward electron beams with characteristic energies of a few
hundred eV to a fewkeV. Two events illustrate the temporal evolution of
bipolar, diverging electric field structures, indicative of positive
U-shaped potentials increasing in magnitude from less than 1kV to a few
kV on a few 100s time scale. This is also the typical formation time for
ionospheric plasma cavities, which are connected to the potential structure
and suggested to evolve hand-in-hand with these. In one of these events an
energy decay of inverted-V ions was observed in the upward field-aligned
current region prior to the acceleration potential increase in the adjacent
downward current region, possibly suggesting that a potential redistribution
took place between the two current branches. The other two events were
characterized by intense unipolar electric fields, indicative of S-shaped
potential contours and were encountered at the polar cap boundary. The total
observation time for these events was typically 10-20s, too short for
monitoring the evolution of the structure, but yet of interest for revealing
their short term stability. The locations of the two bipolar events at the
poleward boundary of the central plasma sheet and of the two unipolar events
at the polar cap boundary, suggest that the special profile shape depends on
whether plasma populations, dense enough to support upward field-aligned
currents and closure of the return current, exist on both sides, or on one
side only, of the boundary. |
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