Simulations examining the structure and powering of the WIM and its connection to H ii regions have also progressed significantly over the past few years. Beyond the continuing increase in computational resources, a resounding theme in recent years has been the impact of density variations on ionized regions. Recognizing that dynamics in either organized or turbulent forms has a considerable impact on the density structure of the ISM, many recent works are finding that such conditions change the ionization and emission of regions to better match recent observations.
Examining the internal structure of locally ionized regions, Giammanco et al. (2004, 2005) and Wood et al. (2005) find that inhomogeneities in H ii regions impact not only the internal ionization and emission structure, but also the amount of ionizing flux that escapes. In the future, these results can be linked more directly to global simulations of the WIM that include input from actual H ii region distributions, such as that of Zurita et al. (2002), discussed in Section 3 above.
At larger scales,
Hill et al. (2008)
find that the emission distribution of the WIM is lognormal, which leads
them to compare the MHD models of a turbulent ISM produced by
Kowal et al. (2007).
A simple, idealized simulation cube allows them to explore a wide range
of parameter space in this study. Simulations that best match the
observed emission distribution are mildly supersonic
(
~ 1.4 - 2.4), producing
line widths consistent with those observed by WHAM. They also find that
the simulated emission distribution is relatively insensitive to changes
in the magnetic field strength.
Using more complex conditions that emulate a slice of the Galaxy, Wood et al. (2010) ionize the supernovae-driven turbulent medium generated in the models of Joung & Mac Low (2006) and Joung, Mac Low, & Bryan (2009). Although the additional complexity does not permit them to study a large parameter space, the attempt to match galactic conditions provides an interesting complement to the more idealized models. They show that with an ionizing source distribution similar to that near the sun, LyC can travel to large distances (> 2 kpc) from the plane. While the details of the gas distributions do not match observations in these non-magnetic simulations, the ability to ionize the WIM through a self-consistent, dynamic ISM is encouraging. Hill et al. (this volume, pg. xxx) discuss this latest work in more detail.