Anisotropic turbulent spectra in the terrestrial magnetosheath as seen by the cluster spacecraft

Here we report the first three-dimensional spatial spectrum of the low frequency magnetic turbulence obtained from the four Cluster spacecraft in the terrestrial magnetosheath close to the magnetopause. We show that the turbulence is compressible and dominated by mirror structures, its energy is injected at a large scale kρ∼0.3 (l∼2000km) via a mirror instability well predicted by linear theory, and cascades nonlinearly and unexpectedly up to kρ∼3.5 (l∼150km), revealing a new power law in the inertial range not predicted by any turbulence theory, and its strong anisotropy is controlled by the static magnetic field and the magnetopause normal.