While more comfortable solutions exist to these two problems today (as described later), the observational findings spawned a number of creative solutions to provide a more extended or even in situ ionization source. Some ideas included:
Although the power requirements still make it hard for one of these options to dominate the ionization of the WIM, many of these sources (save the last) are likely to contribute at some level in spiral galaxies. Some of the variation in line ratios, especially at large distances from the plane, may be due to a higher percentage of flux from these sources in the halo (e.g., Collins & Rand 2001).
At the same time that alternative ionizing sources were being investigated, several groups turned their attention to examining the details of Lyman continuum (LyC) transport. Miller & Cox (1993) describe a scenario where an originally smooth medium populated with opaque clouds is ionized by massive stars to create the WIM from large-scale, overlapping H II regions. Focusing instead on concentrations of massive stars to break out of the disk, Dove & Shull (1994) show that OB associations are density bounded vertically, allowing radiation to escape to large distances. Basu, Johnstone, & Martin (1999) model a specific dynamical solution for the superbubble associated with the W4 H ii region, which Dennison, Topasna, & Simonetti (1997) show has associated faint H. The specific model that explains the dynamical size of the superbubble and the associated emission also allows about 15% of the original ionizing flux to escape vertically from the region. Dove, Shull, & Ferrara (2000) further explore this idea more generally for associations, examining larger vertical regions and exploring various time evolution scenarios for the star formation history.
The collection of these transport studies demonstrated that ionizing flux can be propagated to large distances above the plane with reasonable conditions: either within an association and/or with a specific distribution of the neutral medium. Whether these conditions are justified and globally applicable to explain the pervasive nature of the WIM has only recently been able to be studied in new simulations of the fully multiphase and dynamic ISM (see Section 5).