3D structures of solar wind turbulence from large to small scales

Solar wind is believed to be in a fully-developed turbulent state, but its turbulence properties are different from that of fluid turbulence in many aspects due to the fact that solar wind is a flow of collisionless plasma accompanied by the interplanetary magnetic field. Earlier studies of solar wind turbulence were primarily based on single-spacecraft measurements and the data analyses were performed in the temporal domain. In order to interpret the measured data as spatial variations, assumptions had to be made such as Taylor's frozen-in flow hypothesis and symmetric structures around the mean magnetic field direction or between parallel and anti-parallel directions to the mean field. Four Cluster spacecraft have the potential to determine the three-dimensional spatial properties of solar wind turbulence without assuming Taylor's hypothesis nor any symmetric properties. The wave telescope technique (k-filtering technique) is suitable for such a task. The three-dimensional spatial structures of magnetic field fluctuations are presented on different scales from 10 000 km down to 100 km. Evolution of wave-vector anisotropy is discussed along with energy cascade toward smaller scales.