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| Titel |
Azimuthal velocity shear within an Earthward fast flow – further evidence for magnetotail untwisting? |
| VerfasserIn |
T. Pitkänen, M. Hamrin, P. Norqvist, T. Karlsson, H. Nilsson, A. Kullen, S. M. Imber, S. E. Milan |
| 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 ; 33, no. 3 ; Nr. 33, no. 3 (2015-03-03), S.245-255 |
| Datensatznummer |
250121168
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| Publikation (Nr.) |
 copernicus.org/angeo-33-245-2015.pdf |
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| Zusammenfassung |
| It is well known that nonzero interplanetary magnetic field By conditions lead to a twisted
magnetotail configuration. The plasma sheet is rotated around its axis and
tail magnetic field lines are twisted, which causes an azimuthal displacement
of their ionospheric footprints. According to the untwisting hypothesis, the
untwisting of twisted field lines is suggested to influence the azimuthal
direction of convective fast flows in the nightside geospace. However, there
is a lack of in situ magnetospheric observations, which show actual
signatures of the possible untwisting process. In this paper, we report
detailed Cluster observations of an azimuthal flow shear across the neutral
sheet associated with an Earthward fast flow on 5 September 2001. The
observations show a flow shear velocity pattern with a V⊥y sign
change, near the neutral sheet (Bx~0) within a fast flow during the
neutral sheet flapping motion over the spacecraft. Firstly, this implies that
convective fast flows may not generally be unidirectional across the neutral
sheet, but may have a more complex structure. Secondly, in this event tail
By and the flow shear are as expected by the untwisting hypothesis. The
analysis of the flow shear reveals a linear dependence between Bx and
V⊥y close to the neutral sheet and suggests that Cluster crossed
the neutral sheet in the dawnward part of the fast flow channel. The
magnetospheric observations are supported by the semi-empirical T96 and TF04
models. Furthermore, the ionospheric SuperDARN convection maps support the
satellite observations proposing that the azimuthal component of the
magnetospheric flows is enforced by a magnetic field untwisting. In summary,
the observations give strong supportive evidence to the tail untwisting
hypothesis. However, the T96 ionospheric mapping demonstrates the limitations
of the model in mapping from a twisted tail. |
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