ON MAGNETIC TURBULENCE IN INTERSTELLAR CLOUDS

Observations of diffuse, dark, and giant molecular clouds and their co res are analyzed to determine properties of their turbulent motions. E stimates of characteristic cloud internal density, external extinction , and external radiation held intensity are used to deduce the electro n fraction x(e) due to both photoionization and cosmic rays. This ioni zation fraction exceeds that due to cosmic rays alone, by factors,simi lar to 5 for dark cloud cores to similar to 4000 for giant molecular c louds with embedded OB stars. Estimates of characteristic cloud size, density, velocity dispersion, ionization fraction, and magnetic field strength then indicate that four diagnostic numbers exceed unity by a significant factor: the Reynolds number, the magnetic Reynolds number, the Hartmann number, and the ''wave coupling number,'' or ratio of cl oud size to minimum hydromagnetic wavelength. These results indicate t hat virtually all observed interstellar clouds have strong coupling be tween the magnetic field and the neutral gas, through ion-neutral coll isions, even if the field is weaker than its equipartition value. This coupling allows energetically significant magnetohydrodynamic (MHD) w aves to propagate above cutoff, so that MHD waves, chaotic motions, an d clumpy density structure are probably more pervasive in interstellar clouds than would be expected from cosmic-ray ionization alone. This strong coupling implies that the timescale for ambipolar diffusion is at least similar to 10(7) yr for low-mass cores, and is at least simil ar to 10(8) yr for the gas around cores. These timescales may be too l ong for all of the mass in a low-mass core to condense via ambipolar d iffusion. The observed velocity dispersion is strongly correlated with the estimated electron fraction, according to the power law upsilon s imilar to x(e)p, with p approximate to 0.3. This trend, and those alre ady known among velocity dispersion, size, and density, suggest that i ncreasing extinction may influence the structure of cloud density and velocity dispersion by driving a cycle of decreasing ionization, decre asing MHD wave activity, decreasing velocity dispersion, and increasin g density.