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Azimuthal velocity shear within an Earthward fast flow [Elektronisk resurs] further evidence for magnetotail untwisting?

Pitkänen, Timo 1979- (författare)
Hamrin, Maria (författare)
Norqvist, Patrik (författare)
Karlsson, Tomas (författare)
Nilsson, Hans (författare)
Kullen, Anita (författare)
Imber, Suzanne M. (författare)
Milan, Steve M. (författare)
Umeå universitet Teknisk-naturvetenskapliga fakulteten (utgivare)
Space Physics Group (medarbetare)
Space Physics Group (medarbetare)
Space Physics Group (medarbetare)
2015
Engelska.
Ingår i: Annales Geophysicae. - 0992-7689. ; 33, 245-255
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  • It is well known that nonzero interplanetary magnetic field B y 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 displacementof 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 perpy  sign change, near the neutral sheet ( B x  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 B y and the flow shear are as expected by the untwisting hypothesis. The analysis of the flow shear reveals a linear dependence between B x and V perpy 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. 

Ämnesord

Natural Sciences  (hsv)
Physical Sciences  (hsv)
Fusion, Plasma and Space Physics  (hsv)
Naturvetenskap  (hsv)
Fysik  (hsv)
Fusion, plasma och rymdfysik  (hsv)
rymdfysik  (umu)
Space Physics  (umu)

Indexterm och SAB-rubrik

Magnetospheric physics
magnetospheric configuration and dynamics
magnetotail
plasma convection
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