Alfven vortex filaments observed in magnetosheath downstream of a quasi-perpendicular bow shock

[1] 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-perpendicular bow shock. The turbulent spectrum presents a spectral break accompanied by a broad maximum usually interpreted as due to Alfven ion cyclotron waves. In this paper we establish that this spectral knee corresponds to space-localized coherent magnetic structures in the form of Alfven vortices. The Alfven vortex is a nonlinear cylindrical Alfven wave, quasi-parallel to the mean magnetic field B-0 and propagating in a plane perpendicular to B-0. In this plane the observed vortices are localized within 20c/omega(pi). The frequent observations of such structures indicate their stability in the plasma. Therefore the Alfven vortices can be an important element in the magnetosheath turbulence. The possible origins of these vortices, such as a strong turbulence or the filamentation instability of an Alfven wave, are discussed.