Small Scale Structure and Turbulence in the Interstellar Medium

Summary

I discuss what we have learned about the nature of interstellar turbulence from the technique of radio wave scintillation. My main interest is in the form of the turbulence, i.e. what physical models and equations are appropriate. Radio scintillation observations show that the density irregularities responsible for radio wave scintillation are elongated and probably magnetic-field aligned and characterized by a Kolmogorov spatial power spectrum. It seems reasonable, and almost unavoidable, that the plasma density fluctuations responsible for these scintillations coexist with and are produced by fluctuations in magnetic field and plasma flow velocity which share these properties of the density fluctuations. The main thesis of this paper is that magnetic field and plasma velocity fluctuations with such properties emerge from approximate statements of magnetohydrodynamics such as reduced magnetohydrodynamics or two dimensional magnetohydrodynamics. It is suggested that much insight regarding interstellar magnetohydrodynamics can be gained from study of these relatively simple and intellectually accessible equations.

Introduction

This paper will deal with turbulence in the ionized portion of the interstellar medium. The theoretical ideas invoked will therefore be from the field of plasma turbulence, which in some respects differs from hydrodynamic turbulence. My primary interest, in keeping with the title of this paper, will be in the fluctuations which occur on spatial scales much less than the outer scales of the turbulence, scales which may be termed part of the inertial subrange.