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| Titel |
Discontinuities and Alfvénic fluctuations in the solar wind |
| VerfasserIn |
G. Paschmann, S. Haaland, B. Sonnerup, T. Knetter |
| 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 ; 31, no. 5 ; Nr. 31, no. 5 (2013-05-23), S.871-887 |
| Datensatznummer |
250019040
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| Publikation (Nr.) |
 copernicus.org/angeo-31-871-2013.pdf |
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| Zusammenfassung |
| We examine the Alfvénicity of a set of 188 solar wind directional
discontinuities (DDs) identified in the Cluster data from 2003 by
Knetter (2005), with the objective of separating rotational discontinuities
(RDs) from tangential ones (TDs). The DDs occurred over the full range of
solar wind velocities and magnetic shear angles. By performing the Walén test
in the de Hoffmann–Teller (HT) frame, we show that 77 of the 127 crossings for
which a good HT frame was found had plasma flow speeds exceeding 80% of
the Alfvén speed at an average angular deviation of 7.7°; 33 cases had
speeds exceeding 90% of the Alfvén speed at an average angle of 6.4°.
We show that the angular deviation between flow velocity (in the HT frame)
and the Alfvén velocity can be obtained from a reduced form of the Walén
correlation coefficient. The corresponding results from the Walén test
expressed in terms of jumps in flow speed and corresponding jumps in Alfvén
speed are similar: 66 of the same 127 cases had velocity jumps exceeding
80% with average angular deviation of 5.8°, and 22 exceeding 90% of
the jump in Alfvén speed, with average angular deviation 6.2°. We conclude
that a substantial fraction of the 127 events can be identified as RDs. We
present further evidence for coupling across the DDs by showing that, for
most of the 127 crossings, the HT frame velocities, evaluated separately on
the two sides of the DD, are nearly the same – a result required for RDs but
not for TDs. We also show that the degree of Alfvénicity is nearly the same
for the DDs and fluctuations in which the DDs are embedded. Whatever process
causes deviations from ideal Alfvénicity appears to operate equally for the
DDs as for the surrounding fluctuations. Finally, our study has established a
unique relation between the strahl electron pitch angle and the sign of the
Walén slope, implying antisunward propagation in the plasma frame for all
127 cases. |
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