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| Titel |
Characteristics of the near-tail dawn magnetopause and boundary layer |
| VerfasserIn |
G. Paschmann, S. Haaland, B. U. Ö. Sonnerup, H. Hasegawa, E. Georgescu, B. Klecker, T. D. Phan, H. Rème, A. Vaivads |
| 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 ; 23, no. 4 ; Nr. 23, no. 4 (2005-06-03), S.1481-1497 |
| Datensatznummer |
250015238
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| Publikation (Nr.) |
 copernicus.org/angeo-23-1481-2005.pdf |
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| Zusammenfassung |
| The paper discusses properties of the near-tail dawnside and boundary
layer, as obtained from Cluster plasma and magnetic field measurements during
a single skimming orbit on 4 and 5 July 2001 that included 24 well-defined
crossings by all four spacecraft. As a result of variations of the
interplanetary magnetic field, the magnetic shear across the local varied
between ~0° and ~180°. Using an improved method, which takes
into account acceleration and thickness variation, we have determined the
orientation, speed, thickness and current for the 96 individual
crossings. The orientations show clear evidence of surface waves.
Magnetopause thicknesses range from ~100 to ~2500km, with an
average of 753km. The speeds range from less than 10up to more
than 300, with an average of 48. Both results are consistent with
earlier ISEE and AMPTE results obtained for the dayside magnetopause.
Importantly, scaling the thicknesses to the ion gyro radius or the ion
inertial length did not reduce the large dynamic range. There is also no
significant dependence of thickness on magnetic shear. Current densities
range from ~0.01 up to ~0.3uA, with an average value of 0.05
. By including some extra crossings that did not involve all four
spacecraft, we were able to apply the Walén test to a total of 60 by
Cluster 1 and 3, and have classified 19 cases as rotational discontinuities
(RDs), of which 12 and 7 were sunward and tailward of an X-line,
respectively. Of these 60 crossings, 26 show no trace of a boundary layer.
The only with substantial boundary layers are into the plasma
mantle. Of the 26 without a boundary layer, 8 were identified as RDs.
Since reconnection produces wedge-shaped boundary layers emanating from the
X-line, RDs without may be considered close to the X-line, in
which case the observed magnetic shear and Alfvén Mach number should be
representative of the conditions at the X-line itself. It is therefore
important that four of the eight cases had shear angles ≤100, i.e.
the reconnecting fields were far from being anti-parallel, and that all eight
cases had Alfvén Mach numbers MA>1 in the adjoining magnetosheath.
Another important conclusion can be drawn from the without a that
were tangential discontinuities (TDs). To observe TDs with no at such
large distances from the subsolar point appears to rule out diffusion over
large portions of the as an effective means for plasma transport across
the magnetopause. |
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