Diffraction and refraction of radio waves by irregularities in the interstellar electron density produce a wide range of phenomena that allow inferences of the wavenumber spectrum of the irregularities, anisotropies of the fluctuations, the strength of the variations as a function of location in the Galaxy, and characteristic velocities. I summarize the empirical constraints on these aspects of density microstructure on length scales from about 100 km up to ∼ 1 pc.

Introduction

In this paper I discuss turbulence in the diffuse, ionized component (DIG) of the interstellar medium (ISM). The DIG has been probed on scales that are phenomenally small, by astronomical standards (≳100 km) using innovative radio astronomical techniques. I discuss the propagation effects underlying those techniques and summarize the current state of knowledge about turbulence in the DIG. The information gleaned includes the galactic distribution of the free-electron density, ne, and its fluctuations; the wavenumber spectrum of δne, including the level, shape and extent in wavenumber; and the conclusion that, in addition to the Kolmogorov-like fluctuations that pervade the ISM, there are additional structures on ∼ 1 AU scales but which appear to be independent of the Kolmogorov fluctuations.

Radio-Wave Propagation Effects & Parameters

Radio waves are strongly influenced by the free electron density along the line of sight. Dispersive and scattering effects are determined by the cold-plasma refractive index, nr(v) = (1 − vp2/v2)1/2.