Anisotropy in Space Plasma Turbulence: Solar Wind Observations

The local magnetic field induces many types of anisotropy in plasma turbulence, changing the rate of energy transfer and affecting the propagation of energetic particles. It is challenging to measure this anisotropy in the solar wind due to the limited number of sampling points and measurement difficulties and many aspects remain poorly understood. Nevertheless, in recent years considerable theoretical and experimental progress has been made in understanding the anisotropy of turbulence, the latter through new methods and multi-spacecraft data. A short review of recent work is presented, concentrating on observations rather than theory and discussing the principal limitations and restrictions of such measurements. Key results are discussed: the variation in observed power with angle of the magnetic field to the solar wind flow, and evidence for variations in the spectral index, on scales both above and below the ion gyroradius. A comparison of single and multi-spacecraft analysis methods applied to the same data intervals shows excellent agreement and provides measurements of anisotropy throughout the inertial range. Current outstanding issues are discussed along with their possible resolution.