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Ann. Geophys., 24, 2005-2013, 2006
www.ann-geophys.net/24/2005/2006/
© European Geosciences Union 2006
Cluster observation of plasma flow reversal in the magnetotail during a substorm
A. T. Y. Lui1, Y. Zheng1, Y. Zhang1, S. Livi1, H. Rème2, M. W. Dunlop3, G. Gustafsson4, S. B. Mende5, C. Mouikis6, and L. M. Kistler6
1JHU/APL, Laurel, MD 20723-6099, USA
2CESR, BP4346, 31028 Toulouse Cedex 4, Toulouse, France
3Space Science and Technology Department, RAL, Chilton, Didcot, Oxfordshire OX11 0QX, UK
4Swedish Institute of Space Physics, Uppsala Division, S-755 91 Uppsala, Sweden
5University of California, Space Sciences Laboratory, 7 Gauss Way, Berkeley, CA 94720-7450, USA
6University of New Hampshire, Durham, NH 03824-3525, USA
Abstract. We investigate in detail a reversal of plasma flow from
tailward to earthward detected by Cluster at the downstream distance of
~19 RE in the midnight sector of the magnetotail on
22 August 2001. This flow reversal was accompanied by a sign reversal of the Bz
component and occurred during the late substorm expansion phase as revealed
by simultaneous global view of auroral activity from IMAGE. We examine the
associated Hall current system signature, current density, electric field,
Lorentz force, and current dissipation/dynamo term, the last two parameters
being new features that have not been studied previously for plasma flow
reversals. It is found that (1) there was no clear quadrupole Hall current
system signature organized by the flow reversal time, (2) the x-component of
the Lorentz force did not change sign while the other two did, (3) the
timing sequence of flow reversal from the Cluster configuration did not
match tailward motion of a single plasma flow source, (4) the electric field
was occasionally dawnward, producing a dynamo effect, and (5) the electric
field was occasionally larger at the high-latitude plasma sheet than near
the neutral sheet. These observations are consistent with the current
disruption model for substorms in which these disturbances are due to
shifting dominance of multiple current disruption sites and turbulence at
the observing location.
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