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| Titel |
The exterior cusp and its boundary with the magnetosheath: Cluster multi-event analysis |
| VerfasserIn |
B. Lavraud, T. D. Phan, M. W. Dunlop, M. G. G. G. T. Taylor, P. J. Cargill, J.-M. Bosqued, I. Dandouras, H. Rème, J.-A. Sauvaud, C. P. Escoubet, A. Balogh, A. Fazakerley |
| 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 ; 22, no. 8 ; Nr. 22, no. 8 (2004-09-07), S.3039-3054 |
| Datensatznummer |
250014985
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| Publikation (Nr.) |
 copernicus.org/angeo-22-3039-2004.pdf |
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| Zusammenfassung |
| We report on the observation of three high-altitude cusp crossings by the
Cluster spacecraft under steady northward IMF conditions. The focus of this
study is on the exterior cusp and its boundaries. At the poleward edge of
the cusp, large downward jets are present; they are characterized by a
dawn-dusk component of the convection velocity opposite to the IMF By direction and a gradual evolution (velocity filter effect) corresponding to
an injection site located at the high-latitude magnetopause tailward of the
cusp, with subsequent sunward convection. As one moves from the poleward
edge into the exterior cusp proper, the plasma gradually becomes stagnant as
the result of the mirroring and scattering of the aforementioned plasma
flows. The existence of such a stagnant region (Stagnant Exterior Cusp: SEC) is found in all events
studied here even when the IMF By is large and the clock angle is
~90°.
The SEC-magnetosheath boundary appears as a spatial structure that has a normal
component of the magnetic field pointing inward, in accordance with a
probable connection between the region and the magnetosheath (with northward
field).
This boundary generally has a deHoffmann-Teller velocity that is
slow and oriented sunward and downward, compatible with a discontinuity
propagating from a location near the high-latitude magnetopause. Although
the tangential stress balance is not always satisfied, the SEC-magnetosheath
boundary is possibly a rotational discontinuity. Just outside this boundary,
there exists a clear sub-Alfvénic plasma depletion layer (PDL). These
results are all consistent with the existence of a nearly steady
reconnection site at the high-latitude magnetopause tailward of the cusp. We
suggest that the stability of the external discontinuity (and of the whole
region) is maintained by the presence of the sub-Alfvénic PDL. However,
examination of the electron data shows the presence of heated electrons
propagating parallel to the magnetic field (upward) just outside of the
SEC-magnetosheath boundary. This appears inconsistent with their source
being the northern lobe reconnection site. Finally, the definition of the
magnetopause at high latitudes is revisited. To define the SEC-magnetosheath
boundary as the magnetopause would lead to the misnaming of the "exterior
cusp". |
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