Annu. Rev. Astron. Astrophys. 2004. 42:
211-273
Copyright © 2004 by Annual Reviews. All rights reserved |

Reprinted with kind permission from Annual Reviews, 4139 El Camino Way, Palo Alto, California, USA

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**Abstract.** Turbulence affects the structure and motions of nearly all
temperature and density regimes in the interstellar gas. This
two-part review summarizes the observations, theory, and
simulations of interstellar turbulence and their implications for
many fields of astrophysics. The first part begins with
diagnostics for turbulence that have been applied to the cool
interstellar medium, and highlights their main results. The energy
sources for interstellar turbulence are then summarized along with
numerical estimates for their power input. Supernovae and
superbubbles dominate the total power, but many other sources
spanning a large range of scales, from swing amplified
gravitational instabilities to cosmic ray streaming, all
contribute in some way. Turbulence theory is considered in detail,
including the basic fluid equations, solenoidal and compressible
modes, global inviscid quadratic invariants, scaling arguments for
the power spectrum, phenomenological models for the scaling of
higher order structure functions, the direction and locality of
energy transfer and cascade, velocity probability distributions,
and turbulent pressure. We emphasize expected differences between
incompressible and compressible turbulence. Theories of magnetic
turbulence on scales smaller than the collision mean free path are
included, as are theories of magnetohydrodynamic turbulence and
their various proposals for power spectra. Numerical simulations
of interstellar turbulence are reviewed. Models have reproduced
the basic features of the observed scaling relations, predicted
fast decay rates for supersonic MHD turbulence, and derived
probability distribution functions for density. Thermal
instabilities and thermal phases have a new interpretation in a
supersonically turbulent medium. Large-scale models with
various combinations of self-gravity, magnetic fields, supernovae, and
star formation are beginning to resemble the observed interstellar medium
in morphology and statistical properties. The role of self-gravity
in turbulent gas evolution is clarified, leading to new paradigms
for the formation of star clusters, the stellar mass function, the
origin of stellar rotation and binary stars, and the effects of
magnetic fields. The review ends with a reflection on the progress
that has been made in our understanding of the interstellar
medium, and offers a list of outstanding problems.

**Table of Contents**

- INTRODUCTION
- DIAGNOSTICS OF TURBULENCE IN THE DENSE
INTERSTELLAR MEDIUM
- POWER SOURCES FOR INTERSTELLAR TURBULENCE
- THEORY OF INTERSTELLAR TURBULENCE
- What is Turbulence and Why Is It So Complicated?
- Basic Equations
- Statistical Closure Theories
- Solenoidal and Compressible Modes
- Global Inviscid Quadratic Invariants are Fundamental Constraints on the Nature of Turbulent Flows
- Scale-invariant Kinetic Energy Flux and Scaling Arguments for the Power Spectrum
- Intermittency and Structure Function Scaling
- Details of the Energy Cascade: Isotropy and Independence of Large and Small Scales
- Velocity Probability Distribution
- Turbulent "Pressure"
- Below the Collision Mean Free Path
- MHD Turbulence Theory: Power Spectra
- The Anisotropic Kolmogorov Model

- SIMULATIONS OF INTERSTELLAR TURBULENCE
- Introduction
- Scaling Relations
- Decay of Supersonic MHD Turbulence
- The Density Probability Distribution
- Energy Cascades
- Compressible versus Solenoidal Motions
- Filamentary Structure
- Thermal Instability and Thermal Phases
- Supernova-Driven Turbulence
- The Role of Self-Gravity in the ISM
- Formation of Star Clusters and the IMF
- Rotation and Binary Star Formation
- Effects of Magnetic Fields on Interstellar Turbulence
- Turbulent Rotating Galaxy Disk Simulations

- SUMMARY AND REFLECTIONS
- REFERENCES