Density irregularities in the plasmasphere boundary layer: Cluster observations in the dusk sector

We present observations of plasma density structures crossed by the CLUSTER spacecraft constellation near orbit perigee, with the spacecraft located in the vicinity of the plasmapause boundary. Although the constellation is not arranged as a perfect tetrahedron, the four-point measurements reveal interesting properties of density structures, unknown before the four-points in situ observations from the CLUSTER mission. The set of plasma density profiles shown are derived from the EFW and WHISPER instruments observations in the dusk sector of the plasmasphere region. They yield new insights into: (i) comparative along and cross-field dimensions; (ii) the dynamics of small-scale structures. They illustrate the great opportunity offered by the CLUSTER mission to resolve new challenges in magnetospheric physics, including a clearer understanding of the physics involved in the dynamics of the plasmasphere and the formation of its outer boundary: the plasmapause.