Diagnostics of the Galactic Warm Ionized Medium

Abstract: The Galactic Warm Ionized Medium (WIM) is one of the four atomic phases of the Galaxy, containing most of the ionized gas (and not HII regions). The consensus view is that the WIM is powered by ionizing radiation leaking from HII regions. However, we lack clear understanding of how the ionizing radiation diffuses to great distances. The traditional probes are H-alpha (degree scale; Fabry-Perot) and pulsar dispersion measure (point measurement). Fabry-Perot observations have been conducted in a few optical nebular and auroral lines of metals. Here, I discuss the use of powerful IFUs on ground-based optical telescopes (nebular) to routinely study the WIM on scales of few arcseconds. The MIRI-MRS IFU on JWST allows the WIM to be probed by temperature insensitive mid-iR lines. I will end the talk reviewing an old but important topic: the ionization fraction of H and He in the WIM.