Partially Ionized Gas in the Local Interstellar Medium and Its Galactic Context

Abstract

The most thoroughly observed region of the interstellar medium (ISM) is that which immediately surrounds the heliosphere. Evidence that this gas is partially ionized (about 20% for H, 40% for He) comes from a variety of sources including absorption lines toward nearby stars, and in situ detection of interstellar neutral atoms and pickup ions in the solar system. The ionization in the Local Interstellar Cloud (LIC) is dominated by photoionization and requires more ionizing flux than is available stellar sources. Diffuse radiation from nearby hot gas can provide the necessary hard yet low intensity interstellar radiation field. To put the local ISM in a Galactic context, one can compare it to the warm ionized medium (WIM), but the H ionization level of the LIC is apparently substantially lower than that in the WIM. We discuss the nature and source of ionization of the LIC and compare it with evidence on more distant regions of the warm, diffuse ISM.