This paper briefly reviews a number of fundamental measurements that need to
be made in order to characterize turbulence in space plasmas such as the solar
wind. It has long been known that many of these quantities require simultaneous
multipoint measurements to attain a proper characterization that would reveal
the fundamental physics of plasma turbulence. The solar wind is an ideal plasma
for such an investigation, and it now appears to be technologically feasible to
carry out such an investigation, following the pioneering Cluster and MMS
missions. Quantities that need to be measured using multipoint measurements
include the two-point, two-time second correlation function of velocity,
magnetic field and density, and higher order statistical objects such as third
and fourth order structure functions. Some details of these requirements are
given here, with a eye towards achieving closure on fundamental questions
regarding the cascade rate, spectral anisotropy, characteristic coherent
structures, intermittency, and dissipation mechanisms that describe plasma
turbuelence, as well as its variability with plasma parameters in the solar
wind. The motivation for this discussion is the current planning for a proposed
Helioswarm mission that would be designed to make these measurements,leading to
breakthrough understanding of the physics of space and astrophysical
turbulence.
