Observations of anisotropy in solar wind turbulence. (Invited)

A key property of MHD turbulence is anisotropy caused by the magnetic field. Even if the magnetic field is not strong enough to dominate the thermal pressure, the presence of the field makes fluctuations scale differently in the field-perpendicular direction than in the field-parallel direction. Thus, as the turbulent cascade developes, more energy is transported to wavevectors perpendicular to the magnetic field, than parallel to it. This has strong implications for the scattering of energetic particles and cosmic rays as well as being a useful test for turbulence theories. Over the past five years or so observations and theories about anisotropy have proliferated. This presentation will attempt to summarise recent developments, including: how to measure anisotropy with Fourier transforms, wavelets and structure functions, the key results showing anisotropy in the solar wind, and the implications for testing theories of MHD turbulence. New observations showing anisotropy of Elsasser variables in the solar wind will be presented, these show an unexpected result, that the oppositely directed Elsasser fields have different anisotropy.