On wave modes, structures and turbulence in space plasmas: Cluster results

Magnetic turbulence is known to play a key role in magnetospheric physics since it is involved in all the microphysical processes: energy cascade and dissipation, structure formation, mass transport and particles acceleration, magnetic reconnection, etc. Identifying the experimental properties of turbulence has been therefore one of the major goals of the previous space missions. The results obtained on this issue from the Cluster mission are, certainly, the most accomplished ones. Here we present the last results obtained on the determination of plasma modes and the k-spectra of magnetic turbulence in the terrestrial magnetosheath and in the cusp region using the k-filtering technique and the Cluster data. In the cusp, it is shown that ULF fluctuations are dominated by Alfvénic structures interpreted as KAWs (Kinetic Alfvén Waves), whereas in the magnetosheath they are mainly dominated by compressible mirror structures. By determining their 3D properties, it is shown that turbulence is strongly anisotropic with respect to the quasi-orthogonal directions: the static magnetic field, the magnetopause normal and the plasma flow. Along the flow direction a quasi-1D cascade of the mirror structures is evidenced with a new power law (k_v-8/3), determined over more than one decade. Besides the investigation of the power spectra of turbulence, it is shown that the phase spectra provide crucial information on the presence of coherent/intermittent structures. A method based of surrogate data is presented and a preliminary application on the studied Cluster data is shown. The implications of these new results on both theoretical modelling of space turbulence are also discussed.