[Plasma 2020 Decadal] The essential role of multi-point measurements in turbulence investigations: the solar wind beyond single scale and beyond the Taylor Hypothesis

This paper briefly reviews a number of fundamental measurements that need tobe made in order to characterize turbulence in space plasmas such as the solarwind. It has long been known that many of these quantities require simultaneousmultipoint measurements to attain a proper characterization that would revealthe fundamental physics of plasma turbulence. The solar wind is an ideal plasmafor such an investigation, and it now appears to be technologically feasible tocarry out such an investigation, following the pioneering Cluster and MMSmissions. Quantities that need to be measured using multipoint measurementsinclude the two-point, two-time second correlation function of velocity,magnetic field and density, and higher order statistical objects such as thirdand fourth order structure functions. Some details of these requirements aregiven here, with a eye towards achieving closure on fundamental questionsregarding the cascade rate, spectral anisotropy, characteristic coherentstructures, intermittency, and dissipation mechanisms that describe plasmaturbuelence, as well as its variability with plasma parameters in the solarwind. The motivation for this discussion is the current planning for a proposedHelioswarm mission that would be designed to make these measurements,leading tobreakthrough understanding of the physics of space and astrophysicalturbulence.