& B.G. Elmegreen
Annu. Rev. Astron. Astrophys. 2004. 42:
275-316 Copyright © 2004 by . All rights
One of the most important developments in the field of
interstellar gas dynamics during the last half-century was the
renewed perception that most processes and structures are strongly
affected by turbulence. This is a paradigm shift unparalleled in
many other fields of astronomy, comparable perhaps to the
discovery of extrasolar planets and cosmological structure at high
redshift. Interstellar turbulence and its implications were
commonly discussed in the 1950s, but without the range of
observations that are available today, the theory of the
interstellar medium (ISM) drifted toward increasingly detailed
models based largely on a preference for thermal and dynamical
equilibria. Standing apart were two subfields that continued to
include turbulence as the primary driver of all observations:
radio scintillation and cosmic ray transport. The connections
between these subfields and the prevailing picture of the ISM were
mostly ignored because of the large discrepancy in scales.
Consequently, two other small-scale processes got lost in the
equilibrium paradigm of the large-scale models: mixing of the
elements and molecular chemistry. In this review, we discuss these
four subfields in some detail. All have a well-established
observational base going back several decades, but because of the
complexity of turbulence and the infancy of the relevant theory,
all of them are now in a state of rapid evolution.
Our previous review
Turbulence I (this volume),
emphasized observations in the dense neutral ISM and discussed in
detail the various theoretical approaches to this field.