Warm ionized gas in the local interstellar medium Original Research Article

The properties of widespread ionized gas in the disk and halo of the Milky Way have been explored through the detection of faint optical emission lines from hydrogen, helium, and trace atoms and ions within the gas. Observations reveal extensive regions of warm (T≈104 K), low density (ne∼10−1 cm−3) ionized hydrogen spread throughout the interstellar medium, with enormous loops and filaments stretching across the sky and arching far above the midplane of the Milky Way. These observations have provided new clues about the role of massive stars and supernova explosions in shaping the large scale properties of the interstellar medium as well as the possible relationship between this wide spread gas and the small interstellar cloud surrounding the sun. Observed differences between this local cloud and the more extensive regions of ionization appear to be due primarily to differences in the strength and spectrum of the ionizing radiation field.