CLUSTER is the first space mission dedicated to the three-dimensional study of the terrestrial magnetosphere. Its polar orbit and four spacecraft tetrahedron formation allow it to make in situ measurements in various regions of the magnetosphere, in particular in the plasmasphere. This PhD thesis brings together several studies on plasma structures encountered by the CLUSTER spacecraft along their orbit, during the time period 2001-2004. The physical quantity analysed here is mainly the electronic density, but also the DC magnetic field. In a first part of the thesis, two multipoint analysis tools, the spatial gradient and the time delay, are described, tested - using synthetic datasets then real data - and discussed for typical situations. When conditions are favourable, these analyses reveal the boundary motion and orientation, as well as the dimensions of the structures encountered, impossible to estimate with a single satellite mission. Another part of the thesis deals more globally with the plasmasphere, focusing particularly on the topology and dynamics of small- and large-scale density structures. This study is based on the multipoint analyses previously introduced. It also presents and discusses datasets from other CLUSTER instruments, global images of the plasmasphere from the IMAGE mission, and results from statistical studies and numerical simulations. Finally, the thesis describes a statistical study based on another dataset: the power spectral densities of electromagnetic waves measured at low altitude by the Viking spacecraft in 1986.
