The exterior cusp and its boundary with the magnetosheath: Cluster multi-event analysis
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 d...
| Main Authors: | , , , , , , , , , , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2004-09-01
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| Series: | Annales Geophysicae |
| Online Access: | https://www.ann-geophys.net/22/3039/2004/angeo-22-3039-2004.pdf |
| _version_ | 1819101315577413632 |
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| author | B. Lavraud 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 |
| author_facet | B. Lavraud 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 |
| author_sort | B. Lavraud |
| collection | DOAJ |
| description | 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 <i>B<sub>y</sub></i> 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 <i>B<sub>y</sub></i> 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". |
| first_indexed | 2024-12-22T01:16:43Z |
| format | Article |
| id | doaj.art-474d68cff9cd4107857961c2e42f437f |
| institution | Directory Open Access Journal |
| issn | 0992-7689 1432-0576 |
| language | English |
| last_indexed | 2024-12-22T01:16:43Z |
| publishDate | 2004-09-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Annales Geophysicae |
| spelling | doaj.art-474d68cff9cd4107857961c2e42f437f2022-12-21T18:43:50ZengCopernicus PublicationsAnnales Geophysicae0992-76891432-05762004-09-01223039305410.5194/angeo-22-3039-2004The exterior cusp and its boundary with the magnetosheath: Cluster multi-event analysisB. Lavraud0B. Lavraud1T. D. Phan2M. W. Dunlop3M. G. G. G. T. Taylor4P. J. Cargill5J.-M. Bosqued6I. Dandouras7H. Rème8J.-A. Sauvaud9C. P. Escoubet10A. Balogh11A. Fazakerley12Centre d’Etude Spatiale des Rayonnements, Toulouse, Francenow at: Space and Atmospheric Sciences, Los Alamos National Laboratory, NM, USASpace Science Laboratory, University of California, Berkeley, USARutherford Appleton Laboratory, Didcot, UKSpace and Atmospheric Sciences, Los Alamos National Laboratory, NM, USAImperial College, London, UKCentre d’Etude Spatiale des Rayonnements, Toulouse, FranceCentre d’Etude Spatiale des Rayonnements, Toulouse, FranceCentre d’Etude Spatiale des Rayonnements, Toulouse, FranceCentre d’Etude Spatiale des Rayonnements, Toulouse, FranceESA/ESTEC, Noordwiijk, The NetherlandsImperial College, London, UKMullard Space Science Laboratory, Dorking, Surrey, UKWe 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 <i>B<sub>y</sub></i> 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 <i>B<sub>y</sub></i> 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".https://www.ann-geophys.net/22/3039/2004/angeo-22-3039-2004.pdf |
| spellingShingle | B. Lavraud 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 The exterior cusp and its boundary with the magnetosheath: Cluster multi-event analysis Annales Geophysicae |
| title | The exterior cusp and its boundary with the magnetosheath: Cluster multi-event analysis |
| title_full | The exterior cusp and its boundary with the magnetosheath: Cluster multi-event analysis |
| title_fullStr | The exterior cusp and its boundary with the magnetosheath: Cluster multi-event analysis |
| title_full_unstemmed | The exterior cusp and its boundary with the magnetosheath: Cluster multi-event analysis |
| title_short | The exterior cusp and its boundary with the magnetosheath: Cluster multi-event analysis |
| title_sort | exterior cusp and its boundary with the magnetosheath cluster multi event analysis |
| url | https://www.ann-geophys.net/22/3039/2004/angeo-22-3039-2004.pdf |
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