Abstract

Star-forming galactic discs contain an extended layer of diffuse gas that is photoionised, commonly referred to as the Warm Ionised Medium (WIM) in our own Milky Way, and as the Diffuse Ionised Gas in general. Large Halpha surveys have extensively mapped the properties of the DIG and found typical scale heights of up to ~kpc above the plane of the disc, and high intensities for forbidden line emission from [NII] 6584 and [SII] 6725 that indicate an increase in temperature with height above the disc. To understand these properties and to pinpoint the sources responsible for ionising the DIG , extensive 3D modelling is required. In this talk, I will briefly review our knowledge of the DIG and show how 3D Monte Carlo radiation transfer and (more recently) radiation hydrodynamics models can help us understand (a) the observed ionisation state of the DIG , and (b) the presence of a diffuse layer at the observed heights above the galactic plane