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Ann. Geophys., 24, 735-750, 2006
www.ann-geophys.net/24/735/2006/
© European Geosciences Union 2006
Substorm topology in the ionosphere and magnetosphere during a flux rope event in the magnetotail
O. Amm1, R. Nakamura2, H. U. Frey3, Y. Ogawa4, M. Kubyshkina5, A. Balogh6, and H. Rème7
1Finnish Meteorological Institute, Space Physics program, P.O. Box 503, FIN-00101 Helsinki, Finland
2Space Research Institute, Austrian Academy of Sciences, Schmiedlstraße 6, 8042 Graz, Austria
3University of California, Space Sciences Laboratory, Berkeley, CA 947207450, USA
4SolarTerrestrial Environment Laboratory, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 4648601, Japan
5Institute of Physics, University of St. Petersburg, St. Petersburg 198904, Russia
6Imperial College London, Space & Atmospheric Physics Group, The Blackett Laboratory, London SW7 2BW, UK
7CESR/CNRS, 9, Avenue du Colonel Roche, 31028 Toulouse Cedex 4, France
Abstract. On 13 August 2002, at ~23:00 UT, about 10 min after a substorm
intensification, Cluster observes a flux rope in the central magnetotail,
followed by a localised fast flow event about oneminute later. Associated
with the flux rope event, a traveling compression region (TCR) is seen by
those Cluster spacecraft which reside in the lobe. In the conjugate
ionospheric region in Northern Scandinavia, the MIRACLE network observes the
ionospheric equivalent currents, and the electron densities and electric
fields are measured by the EISCAT radar along a meridional scanning profile.
Further, the auroral evolution is observed with the Wideband Imaging Camera
(WIC) on the IMAGE satellite. We compare in detail the substorm evolution as
observed in the ionosphere and in the magnetosphere, and examine whether
topological correspondences to the flux rope event exist in the ionospheric
signatures. The large-scale mapping of both the location and the
direction of the flux rope to the ionosphere shows an excellent
correspondence to a lens-shaped region of an auroral emission minimum. This
region is bracketed by an auroral region equatorward of it which was
preexisting to the substorm intensification, and a substorm-related
auroral region poleward of it. It is characterised by reduced ionospheric
conductances with respect to its environment, and downward field-aligned
current (FAC) observed both in the magnetosphere and in the ionosphere. As
determined from the ionospheric data, this downward FAC area is moving
eastward with a speed of ~2 km s-1, in good agreement with the
mapped plasma bulk velocity measured at the Cluster satellite closest to
that area. Further southwestward to this leading downward FAC area, a
trailing upward FAC area is observed that moves eastward with the same
speed. The direction of the ionospheric electric field permits a current
closure between these two FAC areas through the ionosphere. We speculate
that these FAC areas may correspond to the ends of the flux rope in its
symmetry direction.
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