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Session 43 - The Diffuse ISM: Milky Way and Beyond.
Display session, Tuesday, June 11
Great Hall,
We propose that low energy cosmic ray electrons can maintain the diffuse warm ionized medium (DWIM), the principal component of the interstellar medium at high Galactic latitudes. We present a model of propagation of these electrons in the vertical direction of the Galactic disk and halo. These electrons lose energy by ionizing and exciting H, He, and He^+, and as heat in the Coulomb scattering by the ambient thermal electrons as well as in the scattering by hydromagnetic waves. The resulting ionization and thermal structure is in good agreement with that of the interstellar medium indicated by pulsar dispersion measures and by observations of diffuse optical and 21 cm emission and ultraviolet absorption. The gas is ionized and heated mainly by the low energy secondary electrons produced as the energetic electrons ionize the gas. Effects of the hydromagnetic waves on the ionization structure are small, especially for clumpy medium. It is predicted that helium is doubly ionized in the DWIM, consistent with the recent negative search for the He I recombination line at 5876 Å\ in the DWIM. The gas is cooled by electron-impact-induced emission of abundant atoms and ions. At small distances from the Galactic midplane, it is mainly by the forbidden line emission of S^+ and by the fine structure line emission of C^+ and Si^+, and at large distances it is dominated by the two photon continuum emission of neutral hydrogen and by the forbidden line emission of C^+, N^+, and O^+. The temperature of the DWIM is determined to be about 10^4 K, in agreement with observations. There appears to be a progression toward reducing clumpiness from the Galactic disk to the halo.
