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| Titel |
More about arc-polarized structures in the solar wind |
| VerfasserIn |
S. Haaland, B. Sonnerup, G. Paschmann |
| 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. 5 ; Nr. 30, no. 5 (2012-05-25), S.867-883 |
| Datensatznummer |
250017228
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| Publikation (Nr.) |
 copernicus.org/angeo-30-867-2012.pdf |
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| Zusammenfassung |
| We report results from a Cluster-based study of the properties of 28
arc-polarized magnetic structures (also called rotational discontinuities) in
the solar wind. These Alfvénic events were selected from the database created
and analyzed by Knetter (2005) by use of criteria chosen to eliminate
ambiguous cases. His studies showed that standard, four-spacecraft timing
analysis in most cases lacks sufficient accuracy to identify the small normal
magnetic field components expected to accompany such structures, leaving
unanswered the question of their existence. Our study aims to break this
impasse. By careful application of minimum variance analysis of the magnetic
field (MVAB) from each individual spacecraft, we show that, in most cases, a
small but significantly non-zero magnetic field component was present in the
direction perpendicular to the discontinuity. In the very few cases where
this component was found to be large, examination revealed that MVAB had
produced an unusual and unexplained orientation of the normal vector. On the
whole, MVAB shows that many verifiable rotational discontinuities (Bn ≠ 0) exist in the solar wind and that their eigenvalue ratio (EVR = intermediate/minimum
variance) can be extremely large (up to EVR = 400). Each
of our events comprises four individual spacecraft crossings. The events
include 17 ion-polarized cases and 11 electron-polarized ones. Fifteen of the
ion events have widths ranging from 9 to 21 ion inertial lengths, with two
outliers at 46 and 54. The electron-polarized events are generally thicker:
nine cases fall in the range 20–71 ion inertial lengths, with two outliers
at 9 and 13. In agreement with theoretical predictions from a
one-dimensional, ideal, Hall-MHD description (Sonnerup et al.,
2010), the
ion-polarized events show a small depression in field magnitude, while the
electron-polarized ones tend to show a small enhancement. This effect was
also predicted by Wu and Lee (2000). Judging only from the sense of the plasma
flow across our DDs, their propagation appears to be sunward as often as
anti-sunward. However, we argue that this result can be misleading as a
consequence of the possible presence of magnetic islands within the DDs. How
the rotational discontinuities come into existence, how they evolve with
time, and what roles they play in the solar wind remain open questions. |
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