DYNAMICAL SIMULATIONS OF PLASMAPAUSE DEFORMATIONS VIVIANE PIERRARD 1,∗ and JUAN CABRERA 2 1 Belgian Institute for Space Aeronomy (IASB-BIRA), 3 av. Circulaire, B-1180 Brussels, Belgium 2 Center for Space Radiations, UCL, 2 Chemin Du Cyclotron, 1348 Louvain-La-Neuve, Belgium ( ∗ Author for correspondence, E-mail: viviane.pierrard@aeronomie.be) (Received 22 September 2005; Accepted in final form: 2 December 2005) Abstract. Dynamical simulations have been developed at IASB-BIRA to model the deformations of the plasmasphere during geomagnetic substorms and other variations in the level of geomagnetic activity. The simulations are based on the mechanism of plasma instability and use the empirical Kp-dependent electric field E5D. The results of the simulations are compared with IMAGE observa- tions that provide the first global comprehensive images of the Earth’s plasmasphere. The predicted plasmapause positions correspond generally rather satisfactorily with the EUV observations. The plasmasphere is rather extended in all MLT sectors during quiet periods. During or just after geo- magnetic substorms, the plasmaspause is sharper and becomes closer to the Earth in the night sector. Periods of enhanced geomagnetic activity are associated to the formation of plumes that rotate with the plasmasphere. The simulations reproduce the formation and the motion of these plumes, as well as the development of other structures like shoulders observed at the plasmapause by EUV on IMAGE. Keywords: plasmasphere, geomagnetic substorms, IMAGE observation 1. Introduction The plasmasphere is the high altitude extension of the low and mid-latitude iono- sphere. To ensure accuracy and reliability of communications, navigation and mil- itary satellites stationed in this region, there is a considerable interest to understand the plasmaspheric environment and its dependence on external parameters. The plasmasphere is filled up by cold plasma distributed along geomagnetic field lines and co-rotating with the Earth. The number density of particles decreases sometimes sharply at the limit of the plasmasphere, called the plasmapause. This discontinuity crosses the geomagnetic equatorial plane at radial distances ranging from 2 Re to 7 Re depending on the geomagnetic activity level. New characteristics of this region of the inner terrestrial magnetosphere have recently been observed by the CLUSTER spacecraft (Darrouzet et al., 2004). New progress in our understand- ing of the plasmaspheric dynamics has also been provided by the observations of EUV (Extreme UltraViolet) on board IMAGE satellite launched in March 2000 (Sandel et al., 2003). After four decades of study of the plasmasphere by in situ measurements of the number density profiles and remote sensing (whistler) tech- niques, IMAGE provides the first global views of the plasmasphere by imaging the distribution of He + in its 30.4 nm resonance line integrated along the line of sight. Space Science Reviews (2006) 122: 119–126 DOI: 10.1007/s11214-006-5670-3 C  Springer 2006