Alfven vortices: magnetosheath Cluster observations and compressible MHD simulations

Magnetic field fluctuations in the frequency range [0.02-12.5] Hz are studied with the four Cluster satellites in the Earth magnetosheath downstream of a quasi-per\-pen\-di\-cu\-lar bow-shock. The turbulent spectrum presents a spectral break accompanied by a broad maximum usually interpreted as due to Alfvén Ion Cyclotron waves. We establish that this spectral knee corresponds to space-localized coherent magnetic structures in the form of Alfvén vortices. The Alfvén vortex is a non-linear cylindrical Alfvén wave, quasi-parallel to the mean magnetic field B0 and propagating in a plane perpendicular to B_0. The frequent observations of such structures indicate their stability in the plasma. To verify the vortex stability, two-dimensional compressible MHD simulations are performed.