  We study the turbulent velocity dispersion spectra of the coexistent HCN and
HCO+ molecular species as a function of length scale in the M17 star-forming
molecular cloud. We show that the observed downward shift of the ion's spectrum
relative to that of the neutral is readily explained by the existence of an
ambipolar diffusion range within which ion and neutral turbulent energies
dissipate differently. We use these observations to evaluate this decoupling
scale and show how to estimate the strength of the plane-of-the-sky component
of the embedded magnetic field in a completely novel way.
