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Ann. Geophys., 19, 1545-1566, 2001
www.ann-geophys.net/19/1545/2001/
© European Geosciences Union 2001
Cluster observations of the high-latitude magnetopause and cusp: initial results from the CIS ion instruments
J. M. Bosqued1, T. D. Phan2, I. Dandouras1, C. P. Escoubet3, H. Rème1, A. Balogh4, M. W. Dunlop4, D. Alcaydé1, E. Amata5, M.-B. Bavassano-Cattaneo5, R. Bruno5, C. Carlson2, A. M. DiLellis5, L. Eliasson6, V. Formisano5, L. M. Kistler7, B. Klecker8, A. Korth9, H. Kucharek8, R. Lundin6, M. McCarthy10, J. P. McFadden2, E. Möbius7, G. K. Parks2, and J.-A. Sauvaud1
1CESR, Toulouse, France
2University of California, Berkeley, CA, USA
3ESTEC/ESA, Noordwijk, the Netherlands
4Blackett Laboratory, Imperial College, London, UK
5IFSI, Rome, Italy
6SISP, Kiruna, Sweden
7University of New Hampshire, Durham, NH, USA
8MPE, Garching, Germany
9MPAe, Lindau, Germany
10University of Washington, Seattle, WA, USA
Abstract. Launched on an elliptical
high inclination orbit (apogee: 19.6 RE) since January 2001
the Cluster satellites have been conducting the first detailed
three-dimensional studies of the high-latitude dayside magnetosphere, including
the exterior cusp, neighbouring boundary layers and magnetopause regions.
Cluster satellites carry the CIS ion spectrometers that provide high-precision,
3D distributions of low-energy (<35 keV/e) ions every 4 s. This paper
presents the first two observations of the cusp and/or magnetopause behaviour
made under different interplanetary magnetic field (IMF) conditions. Flow
directions, 3D distribution functions, density profiles and ion composition
profiles are analyzed to demonstrate the high variability of high-latitude
regions. In the first crossing analyzed (26 January 2001, dusk side, IMF-BZ
< 0), multiple, isolated boundary layer, magnetopause and magnetosheath
encounters clearly occurred on a quasi-steady basis for ~ 2 hours. CIS ion
instruments show systematic accelerated flows in the current layer and adjacent
boundary layers on the Earthward side of the magnetopause. Multi-point analysis
of the magnetopause, combining magnetic and plasma data from the four Cluster
spacecraft, demonstrates that oscillatory outward-inward motions occur with a
normal speed of the order of ± 40 km/s; the thickness of the high-latitude
current layer is evaluated to be of the order of 900–1000 km. Alfvénic
accelerated flows and D-shaped distributions are convincing signatures of a
magnetic reconnection occurring equatorward of the Cluster satellites.
Moreover, the internal magnetic and plasma structure of a flux transfer event
(FTE) is analyzed in detail; its size along the magnetopause surface is ~ 12
000 km and it convects with a velocity of ~ 200 km/s. The second event analyzed
(2 February 2001) corresponds to the first Cluster pass within the cusp when
the IMF-BZ component was northward directed. The analysis of relevant CIS
plasma data shows temporal cusp structures displaying a reverse energy-latitude
"saw tooth" dispersion, typical for a bursty reconnection between the
IMF and the lobe field lines. The observation of D-shaped distributions
indicates that the Cluster satellites were located just a few RE
from the reconnection site.
Key words. Magnetospheric physics
(magnetopause, cusp, and boundary layers; magnetosheath) Space plasma physics
(magnetic reconnection)
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