  Plasma turbulence is investigated using high-resolution ion velocity
distributions measured by the Magnetospheric Multiscale Mission (MMS) in the
Earth's magnetosheath. The particle distribution is highly structured,
suggesting a cascade-like process in velocity space. This complex velocity
space structure is investigated using a three-dimensional Hermite transform
that reveals a power law distribution of moments. In analogy to hydrodynamics,
a Kolmogorov approach leads directly to a range of predictions for this
phase-space cascade. The scaling theory is in agreement with observations,
suggesting a new path for the study of plasma turbulence in weakly collisional
space and astrophysical plasmas.
