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A list of acronyms and terms

a superbubble that extends from the galactic midplane, where it has been energized by supernovae, into the halo. Chimneys can be a conduit for hot gas and hydrogen ionizing radiation.

diffuse ionized gas characterized by temperatures ~ 104 K and densities ~ 10-1 cm-3 that occupies the disk and halo of many spiral galaxies. In our Galaxy the DIG is often referred to as the warm ionized medium or WIM.

dispersion measure; the free electron density integrated along the line of sight to a pulsar; integ0P ne ds. Because the free electron density ne approx nH+ in the interstellar medium, DM is essentially the column density of the H+.

emission measure; the product of the proton density times the electron density integrated along the line of sight. In regions of nearly fully ionized hydrogen, emission measure is essentially integ ne2 ds.

European Photon Imaging Cameras on the XMM-Newton's X-ray telescopes.

far infrared (wavelengths ~ 102  µm).

Forbidden line:
an emission line produced by the decay of an excited metastable state in an atom or ion, denoted by square brackets around the ion's symbol. In the low density WIM/DIG, the collisional de-excitation time scales are much longer than the radiative decay time scales, and therefore the intensity of a forbidden line measures the excitation rate of the metastable state due to collisions by the thermal electrons in the plasma.

the Galaxy:
with a capital `G', our galaxy, the Milky Way.

Galactic latitude:
The angular distance above the Galactic equator, the midplane of the Milky Way.

the emission at 6563 Å produced by the hydrogen Balmer-alpha (n = 3 --> 2) transition following the recombination of ionized hydrogen.

H I region:
a cloud composed primarily of neutral hydrogen atoms.

H II region:
a discrete region of photoionized hydrogen associated with a hot star; a classical "emission nebula" or "Strömgren Sphere", as opposed to the more wide-spread, lower density H+ (the WIM, DIG) not clearly associated with a single, discrete source of ionization.

ionized hydrogen; in this paper ionized hydrogen associated with the wide-spread, low density WIM/DIG.

the Hubble Space Telescope

high velocity cloud; a neutral hydrogen cloud, usually located far from the Galactic disk and not partaking in the rotation of the disk.

Ionization parameter:
the ratio of the ionizing photon density to the electron density, which along with the spectrum, determines the population of ionization states in photoionized gas.

the very local interstellar cloud within a few parsecs of the sun.

Lyman continuum; energies above the Lyman limit, the ionization potential of hydrogen (13.6 eV).

Magellanic Stream:
an extended complex of neutral atomic hydrogen gas associated with a pair of satellite galaxies, the Large and Small Magellanic Clouds, orbiting the Galaxy.

O and B stars:
hot stars that emit Lyman continuum (LyC) photons. Massive O stars are located in active star forming regions, emit a large fraction of their luminosity in the LyC, and are believed to be the primary source of ionization for the WIM/DIG.

polycyclic aromatic hydrocarbon; a large interstellar molecule that can inject heat into the interstellar via its ionization by ultraviolet photons from stars.

rayleigh; a unit of surface brightness for emission lines, equal to 106 / 4pi photons s-1 cm-2 sr-1. An ionized region with an emission measure EM = 2.25 cm-6 pc and a temperature of 8000 K has an Halpha surface brightness of 1 R.

star formation rate.

Spiral galaxy:
a galaxy that has a flat, rotating disk of stars, gas, and dust.

a large cavity of hot (~ 106 K), ionized gas created by the combined kinetic energy of multiple supernovae occurring within an active star formation region.

Te; Ti:
electron temperature and ion temperature, respectively. In general, Te = Ti in the interstellar medium.

turbulent mixing layer; the transition region between two adjacent parts of the interstellar medium that have very different temperatures and are moving with respect to each other.

The Very Large Array radio synthesis telescope near Socorro, New Mexico.

the Wisconsin H-Alpha Mapper; a remotely controlled observatory dedicated to the detection and study of faint emission lines from the interstellar medium of the Galaxy.

the wide-spread, warm, ionized medium in the Galaxy characterized by temperatures ~ 104 K and densities ~ 10-1 cm-3; sometimes also called the "Reynolds Layer".

wide-spread neutral atomic hydrogen characterized by temperatures ~ 103 K and densities ~ 10-1 cm-3.

the Multi-Mirror Mission orbiting X-ray observatory of the European Space Agency.

the perpendicular distance from the midplane of a spiral galaxy.

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