Ambipolar diffusion drifts and dynamos in turbulent gases

Abstract

We consider ambipolar drift in turbulent fluids. Using mean-field electrodynamics, a two-scale theory originally used to study hydromagnetic dynamos, we show that magnetic fields can be advected by small-scale magnetosonic (compressional) turbulence or generated by Alfvénic (helical) turbulence. We make a simple dynamo theory and compare it with standard theories in which dissipation is caused by turbulent diffusion. We also discuss the redistribution of magnetic flux in interstellar clouds. Subject headings: diffusion-hydromagnetics-interstellar: magnetic field-nebulae: internal motions-turbulence I. INTRODUCTION The purpose of this paper is to study some effects of hydromagnetic turbulence in partially ionized astrophysical plasmas. In doing so, we find that two processes which have hitherto been considered separately are actually linked. These processes are ambipolar diffusion-the systematic drift of charged particles and magnetic field lines with respect to neutral atoms-and dynamo action the amplification of magnetic fields by fluid motions. Ambipolar drift can provide relief from the appeal to poorly understood turbulent resistivity that must often be made in astrophysical dynamo theories (see, e.g., Moffatt 1978, Parker 1979, Layzer, Rosner, and Doyle 1979). At the same time, collective or dynamo-like effects can sometimes dominate traditional ambipolar diffusion. Our results have application to the large-scale magnetic field of the galaxy, to interstellar clouds, and possibly to the fields in pre-planetary or other cool circumstellar disks. In § II we develop the basic equations for magnetic fields in turbulent, partially ionized fluids. In § III we discuss dynamo models, and in § IV we discuss the distribution of flux in interstellar clouds. Section V is a summary and discussion.