  Most phases of the interstellar medium contain neutral atoms in addition to
ions and electrons. This introduces differences in plasma physics processes in
those media relative to the solar corona and the solar wind at a heliocentric
distance of 1 astronomical unit. In this paper, we consider two well-diagnosed,
partially-ionized interstellar plasmas. The first is the Diffuse Ionized Gas
(DIG) which is probably the extensive phase in terms of volume. The second is
the gas that makes up the Local Clouds of the Very Local Interstellar Medium
(VLISM). Ion-neutral interactions seem to be important in both media. In the
DIG, ion-neutral collisions are relatively rare, but sufficiently frequent to
damp magnetohydrodynamic (MHD) waves (as well as propagating MHD eddies) within
less than a parsec of the site of generation. This result raises interesting
questions about the sources of turbulence in the DIG. In the case of the VLISM,
the ion-neutral collision frequency is higher than that in the DIG, because the
hydrogen is partially neutral rather than fully ionized. We present results
showing that prominent features of coronal and solar wind turbulence seem to be
absent in VLISM turbulence. For example, ion temperature does not depend on ion
mass. This difference may be attributable to ion-neutral collisions, which
distribute power from more effectively heated massive ions such as iron to
other ion species and neutral atoms.
