The Scale Height and Filling Factor of Warm Ionized Gas in the Milky Way

It is now well established that warm ionized gas is a substantial component of the interstellar medium (ISM) in spiral galaxies. This phase of the ISM can extend many kiloparsecs above the galactic disk, as traced by pulsar dispersion measures in the Milky Way, and by Hα observations of external galaxies. The implied recombination rate requires an enormous reservoir of energy injection, as can only be supplied by the combined photo-ionizing flux of the most massive stars. However, in our own Galaxy, the spatial extent, density and filling factor of warm gas remains unclear. We present a new joint analysis of pulsar dispersion measures, diffuse Hα emission and low frequency synchrotron emission, with which we derive substantially revised estimates of the scale height, density and clumping factor of the warm ionized medium in the Milky Way. These data allow us to trace the change-over from warm to million-degree gas high above the Galactic plane, provide a new approach for estimating pulsar distances at high Galactic latitudes, and can be combined with Faraday rotation measurements to map out the magnetic field of the Galactic halo.