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Volume 19, issue 10/12
Ann. Geophys., 19, 1545–1566, 2001
https://doi.org/10.5194/angeo-19-1545-2001
© Author(s) 2001. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: CLUSTER

Ann. Geophys., 19, 1545–1566, 2001
https://doi.org/10.5194/angeo-19-1545-2001
© Author(s) 2001. This work is distributed under
the Creative Commons Attribution 3.0 License.

  30 Sep 2001

30 Sep 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 J. M. Bosqued et al.
  • 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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