ANGEO Annales Geophysicae ANGEO 1432-0576 Copernicus GmbH Göttingen, Germany 10.1007/s00585-000-0411-5 A case study of a radially polarized Pc4 event observed by the Equator-S satellite Cramm R. 1 4 5 Glassmeier K. H. 1 4 5 Othmer C. 1 4 5 Fornacon K. H. 1 4 5 Auster H. U. 1 4 5 Baumjohann W. 2 4 5 Georgescu E. 2 3 4 5 Institut für Geophysik und Meteorologie, Technische Universität, Mendelssohnstr. 3, D-38106 Braunschweig, Germany Max-Planck-Institut für extraterrestrische Physik, PO Box 1603, D-85740 Garching, Germany Institute of Space Sciences, Bucharest, Romania <i>Correspondence to:</i> C. Othmer e-mail: c.othmer@tu-bs.de 30 11 1999 18 4 411 415 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/18/411/2000/angeo-18-411-2000.html The full text article is available as a PDF file from http://www.ann-geophys.net/18/411/2000/angeo-18-411-2000.pdf

A 16 mHz Pc4 pulsation was recorded on March 17, 1998, in the prenoon sector of the Earth's magnetosphere by the Equator-S satellite. The event is strongly localized in radial direction at approximately <i>L</i> = 5 and exhibits properties of a field line resonance such as an ellipticity change as seen by applying the method of the analytical signal to the magnetic field data. The azimuthal wave number was estimated as <i>m</i> \approx 150. We discuss whether this event can be explained by the FLR mechanism and find out that the change in ellipticity is more a general feature of a localized Alfvén wave than indicative of a resonant process.<br><br><b>Key words:</b> Magnetospheric physics (MHD waves and instabilities)