An Observational Method to Measure the Relative Fractions of Solenoidal
  and Compressible Modes in Interstellar Clouds
  We introduce a new method for observationally estimating the fraction of
momentum density (${\rho}{\mathbf{v}}$) power contained in solenoidal modes
(for which $\nabla \cdot {\rho}{\mathbf{v}} = 0$) in molecular clouds. The
method is successfully tested with numerical simulations of supersonic
turbulence that produce the full range of possible solenoidal/compressible
fractions. At present the method assumes statistical isotropy, and does not
account for anisotropies caused by (e.g.) magnetic fields. We also introduce a
framework for statistically describing density--velocity correlations in
turbulent clouds.
