On Scaling Laws and Alfvénic Magnetic Fluctuations in Molecular Clouds

Under the basic assumption that the observed turbulent motions in molecularclouds are Alfvenic waves or turbulence, we emphasize that the Dopplerbroadening of molecular line profiles directly measures the velocity amplitudesof the waves instead of the Alfven velocity. Assuming an equipartition betweenthe kinetic energy and the Alfvenic magnetic energy, we further propose thehypothesis that observed standard scaling laws in molecular clouds imply aroughly scale-independent fluctuating magnetic field, which might be understoodas a result of strong wave-wave interactions and subsequent energy cascade. Wepredict that $\sigma_{v}\propto \rho^{-0.5}$ is a more basic and robustrelation in that it may approximately hold in any regions where the spatialenergy density distribution is primarily determined by wave-wave interactions,including gravitationally unbound regions. We also discuss the fact that ascale-independent $\sigma_{B}^{2}$ appears to contradict existing 1-D and 2-Dcomputer simulations of MHD turbulence in molecular clouds.Comment: 11 pages, to appear in Astrophysical Journal Letters, email: tao@astro.umd.ed