Multi-Spacecraft Observation Of Small-Scale Anisotropy In The Solar Wind Turbulence

We study the anisotropy of small-scale magnetic field turbulence in the solar wind using Cluster multipoint observations. In the classical Kolmogorov theory of neutral fluid turbulence, the statistical isotropy is restored at the small-scales. However, recent experiments and simulations show that in magnetohydrodynamical fluids the anisotropy of turbulence due to a background magnetic field increases down to fairly small scales. Conventionally, turbulence is studied by looking at the scaling properties of the structure functions, i.e. the moments of the magnetic field or velocity differences measured at two points separated by some distance. So far, the single spacecraft measurements allowed their calculation in only one spatial direction that of the plasma flow. Using pairs of Cluster spacecraft we can analyze the structure functions of the solar wind magnetic field at different angles relative to the mean magnetic field direction. This allows investigating the degree to which small scales are isotropic or anisotropic in different spatial directions.