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\subsubsection{Spatial Power Spectrum}\label{ps}  {\bf Switch order with VCA}  The Fourier power spectrum is one of the most widely computed turbulent statistics. Numerical simulations over the last decade have confirmed that the velocity power spectral slope is $P_v(k) \propto k^{-2}$ for supersonically turbulent gas \citep{cite review}. The slope is similar for a magnetized gas where the gas and field are well-coupled \citep[e.g.][and references therein]{maclow04,MandO07}. The power spectrum of the 3D density distribution of turbulent gas is $P_\rho(k) \propto k^{}$ \citep{federrath07}. Observationally, the situation is more complex since the intensity distribution in a spectral line cube is a product of both density and velocity fluctuations, which are inextricably entangled. For lower density tracers, like $^{12}$CO, the gas becomes optically thick and emission saturates along high-density sight-lines through the cloud. \citep{lazarian04} predicted that the intensity power spectrum intensity field $P(k) \propto k^{-11/3}$ and saturates to $P(k) \propto k^{-3}$ in the optically thick limit. This was confirmed in numerical simulations by \citep{burkhart13}, who post-processed the simulations to produce synthetic CO maps in different optical depth regimes. Because the emission behaves differently in different optical depth limits, it is possible to probe the underlying density and velocity slopes by analyzing the spectrum of different slices within the spectral cube \citep{lazarian00}, a technique that we discuss further in \S\ref{VCA}.