A list of acronyms and terms

Chimney:

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.

DIG:

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.

DM:

dispersion measure; the free electron density integrated along the line of sight to a pulsar; ₀^P n_e ds. Because the free electron density n_e n_H⁺ in the interstellar medium, DM is essentially the column density of the H⁺.

EM:

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 n_e² ds.

EPIC:

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

FIR:

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.

H:

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.

H⁺:

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

HST:

the Hubble Space Telescope

HVC:

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.

LIC:

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

LyC:

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.

PAH:

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

R:

rayleigh; a unit of surface brightness for emission lines, equal to 10⁶ / 4 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 H surface brightness of 1 R.

SFR:

star formation rate.

Spiral galaxy:

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

Superbubble:

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.

T_e; T_i:

electron temperature and ion temperature, respectively. In general, T_e = T_i in the interstellar medium.

TML:

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.

VLA:

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

WHAM:

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.

WIM:

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".

WNM:

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

XMM-Newton:

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

z:

the perpendicular distance from the midplane of a spiral galaxy.