Density structures inside the plasmasphere: Cluster observations F. Darrouzet, P. M. E. Decreau, J. De Keyser, A. Masson, D. L. Gallagher, O. Santolik, B. R. Sandel, J. G. Trotignon, J. L. Rauch, E. Le Guirriec, P. Canu, F. Sedgemore, M. Andre, and J. F. Lemaire The electron density profiles derived from the EFW and WHISPER instruments on board the four Cluster spacecraft reveal density structures inside the plasmasphere and at its outer boundary, the plasmapause. We have conducted a statistical study to characterize these density structures. We focus on the plasmasphere crossing on 11April 2002, during which Cluster observed several density irregularities inside the plasmasphere, as well as a plasmaspheric plume. We derive the density gradient vectors from simultaneous density measurements by the four spacecraft. We also determine the normal velocity of the boundaries of the plume and of the irregularities from the time delays between those boundaries in the four individual density profiles, assuming they are planar. These new observations yield novel insights about the occurrence of density irregularities, their geometry and their dynamics. These in-situ measurements are compared with global images of the plasmasphere from the EUV imager on board the IMAGE satellite. Key words. Magnetospheric physics (plasmasphere)  Space plasma physics (general or miscellaneous) phere, the plasmapause, has been discovered independently from ground-based whistler wave observations (Carpenter, 1963) and from in situ satellite measurements (Gringauz, 1963). It is often characterized by a sharp decrease in the plasma density, from 100 1000 particles/cm 3 down to a few particles/cm 3. This knee is formed at an equatorial distance ranging from 2 to 8 Earth radii, depending on the level of the geomagnetic activity. The Cluster mission is the first multi-spacecraft mission that surveys the plasmasphere. This paper focuses on Cluster observations of density structures. Before the Cluster mission, large fluctuations in the electron number density had been observed by OGO 5 (Chappell et al., 1970a), by CRRES near the plasmapause (LeDocq et al., 1994), by magnetospheric plasma analyzers on board geosynchronous satellites (Moldwin et al., 1995), and by various other groundbased and spacecraft instruments (see review by Carpenter and Lemaire, 1997). Density structures had also been inferred from high-resolution magnetometer data obtained during the Earth swing-by of Cassini (Southwood et al., 2001). ________________ Annales Geophysicae 22: 2577Ð2585, 2004