Analysis of plasmaspheric plumes: CLUSTER and IMAGE observations and numerical simulations F. Darrouzet, J. De Keyser, P.M.E. Decreau, D.L. Gallagher, V. Pierrard, J.F. Lemaire, B.R. Sandel, I. Dandouras, H. Matsui, M. Dunlop, J. Cabrera, A. Masson, P. Canu, J.G. Trotignon, J.L. Rauch, and M. Andre Plasmaspheric plumes have been routinely observed by CLUSTER and IMAGE. The CLUSTER mission provides high time resolution four-point measurements of the plasmasphere near perigee. Total electron density profiles have been derived from the electron plasma frequency identified by the WHISPER sounder supplemented, in-between soundings, by relative variations of the spacecraft potential measured by the electric field instrument EFW; ion velocity is also measured onboard these satellites. The EUV imager onboard the IMAGE spacecraft provides global images of the plasmasphere with a spatial resolution of 0.1 RE every 10 minutes; such images acquired near apogee from high above the pole show the geometry of plasmaspheric plumes, their evolution and motion. We present coordinated observations of three plume events and compare CLUSTER in-situ data with global images of the plasmasphere obtained by IMAGE, and together with numerical simulations of the formation of plumes. These simulations are based on a model that includes the interchange instability mechanism and the McIlwain E5D electric field model. In particular, we study the geometry and the orientation of plasmaspheric plumes by using a four-point analysis method: the spatial gradient. We also compare several aspects of their motion as determined by different methods: (i) inner and outer plume boundary velocity calculated from time delays of this boundary observed by the wave experiment WHISPER on the four spacecraft, (ii) ion velocity derived from the ion spectrometer CIS onboard CLUSTER, (iii) drift velocity measured by the electron drift instrument EDI onboard CLUSTER and (iv) global velocity determined from successive EUV images. These different techniques consistently indicate that plasmaspheric plumes rotate around the Earth, with their foot fully co-rotating, but with their tip rotating slower and moving farther out. This result is consistent with recent studies. However, the systematic outward motion of the plume found is in contradiction with standard MHD convection scenarios, based on the enhancement of a uniform dawn-dusk convection electric field, considered in earlier teardrop models of the plasmasphere. For all cases, numerical simulations reproduce the formation and motion of the plasmaspheric plumes when the level of geomagnetic activity increases suddenly by a sufficiently large step (_K greater than or equal to 2). _______________ Submitted to Annalles Geophysicae, 2005