Annales Geophysicae www.ann-geophys.net 0992-7689 1432-0576 26 11 2008 10.5194/angeo-26-3365-2008 http://www.ann-geophys.net/26/3365/2008/ http://www.ann-geophys.net/26/3365/2008/angeo-26-3365-2008.html http://www.ann-geophys.net/26/3365/2008/angeo-26-3365-2008.pdf 3365 3373 2008-10-21 Transients in oxygen outflow above the polar cap as observed by the Cluster spacecraft H. Nilsson hans.nilsson@irf.se M. Waara O. Marghitu M. Yamauchi R. Lundin H. Rème J.-A. Sauvaud I. Dandouras E. Lucek L. M. Kistler B. Klecker C. W. Carlson M. B. Bavassano-Cattaneo A. Korth Swedish Institute of Space Physics, Kiruna, Sweden Max-Planck-Institut für Extraterrestriche Physik, Garching, Germany Institute for Space Sciences, Bucharest, Romania Centre d'Etude Spatiale des Rayonnements, Toulouse, France Imperial College of Science, Technology and Medicine, London, UK University of New Hampshire, Durham, USA Space Science Laboratory, University of California, Berkeley, USA Istituto di Fisica dello Spazio Interplanetario, Rome, Italy Max-Planck-Institut für Sonnensystemforschung, Katlenburg-Lindau, Germany Oxygen ion outflow associated with the cusp and cleft give rise to persistent oxygen ion beams which can be observed over the polar cap. For high altitude spacecraft such as Cluster these beams are often observed for several hours on each occasion. This allows for a study of typical temporal structures on the time scale of minutes. We have used 3 years of data from spring, January to May of years 2001 to 2003, for a study of the oxygen number flux variation in the polar cap ion outflow. The source of these oxygen ion beams is the cusp and cleft, and variations in ionospheric upflow on time scales of around 8 min have been reported from ground based studies using incoherent scatter radar. Such upflows typically do not reach escape velocity, and further energization above the ionosphere is required for outflow to occur. Our study shows that a typical time scale between sudden number flux enhancements observed by Cluster in a geocentric distance range of 5 <I>R_E</I> to 12 <I>R_E</I> is 5 to 10 min. A superposed epoch study does not reveal any significant convection velocity or temperature changes around the flux enhancement events. 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