The warm ionized medium

The warm ionized component of the interstellar medium is characterized by widespread regions of nearly fully ionized hydrogen with an electron density ne≂0.1 cm−3 and a temperature T≂8000 K. The regions occupy at least 20% of the volume within a layer of half‐thickness H≂1 kpc and have a mass surface density approximately one‐third that of the H I. The required ionizing power is 1×10−4 ergs s−1 per cm2 of the Galactic disk near the solar circle. These results have been derived from a relatively small number of sight lines that sample a region of radius 3‐4 kpc about the sun. The temperature and ionization state have been probed at low Galactic latitudes near the midplane, while the scale height, density, filling fraction, and power requirement are derived from data at high latitudes.