The existence of substantial ionized gas outside of classical H ii regions was first suggested by Hoyle & Ellis (1963). Very low frequency radio observations of the Galactic synchrotron emission show a turnover below 10 MHz, a marked deviation from a constant power law at higher frequencies. The peak emission in the spectrum changes with latitude, with the highest latitudes having the lowest frequency turnover. Hoyle & Ellis interpreted this break from the power law relationship as evidence for free-free absorption of the synchrotron emission by electrons along the line of sight. With a clear lack of discrete ionized regions at high latitudes, they posited a diffuse ionized layer must exist to provide the absorption. Using the latitude dependence of the peak shift and plausible arguments about the temperature and density of the absorbing layer, they present a model of the WIM with physical characteristics only a factor of a few from adopted values today.
Shortly following their work, the discovery of pulsars (Hewish et al. 1968) afforded a direct measure of the free electron column along lines of sight since their dispersion measure (DM ne dl) causes a time delay in pulse arrival as a function of frequency. Significant dispersion measures are seen toward pulsars - especially those above the plane - that have no obvious ionized region along the line of sight.
While both of these observations from the radio sky provide strong evidence for ionized gas far from classical H ii regions, they are difficult to fully examine the physical characteristics of the WIM. Instead, investigators pushed hard to detect direct emission from the WIM, most notably in the optical where the predicted densities and temperatures of the gas were expected to produce a very faint spectrum similar to H ii regions, including the typical hydrogen recombination lines. Sivan (1974) presented a deep, wide-field H survey that revealed substantially fainter ionized regions than had been previously seen, including faint, extended structure near several nearby H ii regions. However, detection of the WIM in moderate-resolution imaging surveys was not possible until the advent of modern CCD detectors (see Section 4).
In the meantime, Reynolds et al. (1973a, b) utilized large-format (15 cm) Fabry-Perot etalons to detect and resolve spectral line emission from the ISM more than an order of magnitude fainter than the sensitivity limits of direct imaging at the time. Leveraging the ability of such spectrometers to accept a large solid angle, Reynolds traded spatial resolution (~1deg) for sensitivity and spectral resolution and began an observational campaign to fully explore the nature of this extended, pervasive ionized component of the ISM.
After about two decades of dedicated work, Reynolds and collaborators had established the basic characteristics of the WIM in the Milky Way:
This power rate is quite high (Reynolds 1984). In comparison to some well-known sources, the observed recombination rate requires nearly all the mechanical energy output from supernovae in the Galaxy or about 15% of the total ionizing flux of massive (OB) stars. While the latter provides a much more comfortable margin and a natural extension of their obvious H ii regions, there are two immediate problems that need to be explained.
First, if massive stars are the source, we must be able to explain how at least 15% of their ionizing flux escapes to these large distances above the plane. With their short lifetimes and the concentration of massive star-forming regions close to the plane, few of these stars travel far from the dense medium of their birth. Combined with the large cross section of neutral hydrogen, simple distributions of the global ISM do not allow this population to contribute much at large distances. Some early attempts to add realistic complexity to models are discussed in Section 2, while more recent efforts leveraging our tremendous increase in resources today are reviewed in Section 5.
Second, while the WIM does emit optical forbidden line emission similar to O-star H ii regions, it has characteristically lower [O iii] / H and He i / H ratios as well as higher [N ii] / H and [S ii] / H. The simplest interpretation of these differences is that lower ionization states are maintained in the WIM; the dominant ions are N+, S+, and O+. Such a result could be explained by a softer ionizing spectrum, however as ionizing flux is absorbed by neutral hydrogen away from the source it typically hardens; photons near 13.6 eV are preferentially absorbed. More likely, as shown by Domgorgen & Mathis (1994), the gas can equilibrate in these lower states if the photon to gas density ratio (the ionization parameter) is very low. In such a dilute radiation field, atoms have time to recombine on average before the next ionizing photon arrives.