ANGEO Annales Geophysicae ANGEO 1432-0576 Copernicus GmbH Göttingen, Germany 10.5194/angeo-22-3707-2004 Cluster observations of a complex high-altitude cusp passage during highly variable IMF Taylor M. G. G. T. 1 2 Dunlop M. W. 3 Lavraud B. 1 4 Vontrat-Reberac A. 5 Owen C. J. 2 Décréau P. 6 Trávnícek P. 7 Elphic R. C. 1 Friedel R. H. W. 1 Dewhurst J. P. 2 Wang Y. 1 Fazakerley A. 2 Balogh A. 8 Rème H. 4 Daly P. W. 9 Space and Atmospheric Sciences, Los Alamos National Laboratory, New Mexico, USA Mullard Space Science Laboratory, University College London, UK Rutherford Appleton Laboratory, Didcot, Oxfordshire, UK Centre d’Etude Spatiale des Rayonnements, Toulouse, France Centre d’étude des Environnements Terrestre et Planétaires, Velizy, France LPCE/CNRS 45071 Orleans Cedex 2, France Institute of Atmospheric Physics, The Academy of Sciences of the Czech Republic Imperial College of Science Technology and Medicine, London, UK Max-Planck-Institut für Aeronomie, D-37191 Katlenburg-Lindau, Germany Republic 03 11 2004 22 10 3707 3719 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.ann-geophys.net/22/3707/2004/angeo-22-3707-2004.html The full text article is available as a PDF file from http://www.ann-geophys.net/22/3707/2004/angeo-22-3707-2004.pdf

On 26 February 2001, the Cluster spacecraft were outbound over the Northern Hemisphere, at approximately 12:00&nbsp;MLT, approaching the magnetosheath through the high-altitude (and exterior) cusp region. Due to macroscopic motions of the cusp, the spacecraft made multiple entries into the exterior cusp region before exiting into the magnetosheath, presenting an excellent opportunity to utilize the four spacecraft techniques available to the Cluster mission. We present and compare 2 methods of 4-spacecraft boundary analysis, one using PEACE data and one using FGM data. The comparison shows reasonable agreement between the techniques, as well as the expected "single spacecraft" plasma and magnetic signatures when associated with propagated IMF conditions. However, during periods of highly radial IMF (predominantly negative <i>B<sub>X</sub></i> GSM), the 4-spacecraft boundary analysis reveals a dynamic and deformed cusp morphology.