The Wisconsin H-Alpha Mapper (WHAM) has surveyed the distribution and kinematics of ionized gas in the Galaxy above declination −30 • . The WHAM -2 -Northern Sky Survey (WHAM-NSS) has an angular resolution of one degree and provides the first absolutely-calibrated, kinematically-resolved map of the Hα emission from the Warm Ionized Medium (WIM) within ∼ ±100 km s −1 of the Local Standard of Rest. Leveraging WHAM's 12 km s −1 spectral resolution, we have modeled and removed atmospheric emission and zodiacal absorption features from each of the 37,565 spectra. The resulting Hα profiles reveal ionized gas detected in nearly every direction on the sky with a sensitivity of 0.15 R (3σ). Complex distributions of ionized gas are revealed in the nearby spiral arms up to 1-2 kpc away from the Galactic plane. Toward the inner Galaxy, the WHAM-NSS provides information about the WIM out to the tangent point down to a few degrees from the plane. Ionized gas is also detected toward many intermediate velocity clouds at high latitudes. Several new H II regions are revealed around early B-stars and evolved stellar cores (sdB/O). This work presents the details of the instrument, the survey, and the data reduction techniques. The WHAM-NSS is also presented and analyzed for its gross properties. Finally, some general conclusions are presented about the nature of the WIM as revealed by the WHAM-NSS.

This article reviews observations and models of the diffuse ionized gas that permeates the disk and halo of our Galaxy and others. It was inspired by a series of invited talks presented during an afternoon scientific session of the 65th birthday celebration for Professor Carl Heiles held at Arecibo Observatory in August 2004. This review is in recognition of Carl's long standing interest in and advocacy for studies of the ionized as well as the neutral components of the interstellar medium.Comment: 29 pages, 19 figures; accepted by Reviews of Modern Physic

We report on observations of several optical emission lines toward a variety of newly revealed faint, large-scale H -emitting regions in the Galaxy. The lines include [, and He i k5876 obtained with the Wisconsin H Mapper (WHAM ) toward sight lines that probe superbubbles, high-latitude filamentary features, and the more diffuse warm ionized medium ( WIM). Our observations include maps covering thousands of square degrees toward the well-known Orion-Eridanus bubble, a recently discovered 60; 20 bipolar superbubble centered in Perseus, plus several classical H ii regions surrounding OB stars and hot evolved stellar cores. We use the emission-line data to explore the temperature and ionization conditions within the emitting gas and their variations between the different emission regions. We find that in the diffuse WIM and in the faint high-latitude filamentary structures the line ratios of [ N ii]/H and [S ii]/H are generally high, while [O iii]/ H and He i/H are generally low compared to the bright classical H ii regions. This suggests that the gas producing this faint widespread emission is warmer, in a lower ionization state, and ionized by a softer spectrum than gas in classical H ii regions surrounding O stars, the presumed ionization source for the WIM. In addition, we find differences in physical conditions between the large bubble structures and the more diffuse WIM, suggesting that the ionization of superbubble walls by radiation from interior O associations does not account entirely for the range of conditions found within the WIM, particularly the highest values of [ N ii]/H and [S ii]/ H .

A large portion of the Galaxy (ℓ = 123 • to 164 • , b = −6 • to −35 • ), which samples regions of the Local (Orion) spiral arm and the more distant Perseus arm, has been mapped with the Wisconsin H-Alpha Mapper (WHAM) in the [S II] λ6716 and [N II] λ6583 lines. By comparing these data with the maps from the WHAM Hα Sky Survey, we begin an investigation of the global physical properties of the Warm Ionized Medium (WIM) in the Galaxy. Several trends noticed in emission-line investigations of diffuse gas in other galaxies are confirmed in the Milky Way and extended to much fainter emission. We find that the [S II]/Hα and [N II]/Hα ratios increase as absolute Hα intensities decrease. For the more distant Perseus arm emission, the increase in these ratios is a strong function of Galactic latitude, b, and thus, of height, z, above the Galactic plane, while the [S II]/[N II] ratio is relatively independent of Hα intensity. Scatter in this ratio appears to be physically significant, and maps of [S II]/[N II] suggest regions with similar ratios are spatially correlated. The Perseus arm [S II]/[N II] ratio is systematically lower than Local emission by 10%-20%. With [S II]/[N II] fairly constant over a large range of Hα intensities, the increase of [S II]/Hα and [N II]/Hα with |z| seems to reflect an increase in temperature. Such an interpretation allows us to estimate the temperature and ionization conditions in our large sample of observations. We find that WIM temperatures range from 6,000 K to 9,000 K with temperature increasing from bright to faint Hα emission (low to high [S II]/Hα and [N II]/Hα) respectively. Changes in [S II]/[N II] appear to reflect changes in the local ionization conditions (e.g. the S + /S ++ ratio). We also measure the electron scale height in the Perseus arm to be 1.0 ± 0.1 kpc, confirming earlier, less accurate determinations.

Dynamic interactions between the two Magellanic Clouds have flung large quantities of gas into the halo of the Milky Way. The result is a spectacular arrangement of gaseous structures including the Magellanic Stream, the Magellanic Bridge, and the Leading Arm (collectively referred to as the Magellanic System). In this third paper of a series studying the Magellanic gas in absorption, we analyze the gas ionization level using a sample of 69 Hubble Space Telescope/Cosmic Origins Spectrograph sightlines that pass through or within 30 • of the 21 cm-emitting regions. We find that 81% (56/69) of the sightlines show UV absorption at Magellanic velocities, indicating that the total cross section of the Magellanic System is ≈11 000 square degrees, or around a quarter of the entire sky. Using observations of the Si III/Si II ratio together with Cloudy photoionization modeling, we calculate the total gas mass (atomic plus ionized) of the Magellanic System to be ≈2.0×10 9 M ⊙ (d/55 kpc) 2 , with the ionized gas contributing around three times as much mass as the atomic gas. This is larger than the current-day interstellar H I mass of both Magellanic Clouds combined, indicating that they have lost most of their initial gas mass. If the gas in the Magellanic System survives to reach the Galactic disk over its inflow time of ∼0.5-1.0 Gyr, it will represent an average inflow rate of ∼3.7-6.7 M ⊙ yr −1 , potentially raising the Galactic star formation rate. However, multiple signs of an evaporative interaction with the hot Galactic corona indicate that the Magellanic gas may not survive its journey to the disk fully intact, and will instead add material to (and cool) the corona.