Multi-point measurements of turbulence in space plasma

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Near-Earth space such as the solar wind and the terrestrial magnetosphere serves as a natural laboratory for studying fundamental properties of plasma turbulence, and has important implications to astrophysical applications. While earlier spacecraft observations revealed that magnetic field fluctuations are reminiscent of turbulence in these regions, the observations were primarily limited to single-spacecraft measurements and separation between spatial structures and temporal variations has always been a matter of debate in interpreting the data. With the advent of the Cluster mission, the four-spacecraft mission, it has become possible to determine the three-dimensional spatial structures of plasma turbulence directly in near-Earth space. Analysis methods of plasma turbulence are developed for multi-spacecraft measurements and applied to the Cluster data for the first time. The multi-point analysis methods enable us to investigate fluctuation properties not only in frequency but also in wave vector domain, visualizing symmetric properties of turbulence and thus opening a new gate for studying turbulence. Fundamental questions are spotlighted on the nonlinear nature of turbulence, which are challenges in both theoretical and experimental physics.