WHAM Observations of Hα, [Sii], and [Nii] toward the Orion and Perseus Arms: Probing the Physical Conditions of the Warm Ionized Medium

A large portion of the Galaxy (l = 123 deg to 164 deg, b = -6 deg to -35deg), which samples regions of the Local (Orion) spiral arm and the moredistant Perseus arm, has been mapped with the Wisconsin H-Alpha Mapper (WHAM)in the H-Alpha, [S II] 6716, and [N II] 6583 lines. Several trends noticed inemission-line investigations of diffuse gas in other galaxies are confirmed inthe Milky Way and extended to much fainter emission. We find that the [SII]/H-Alpha and [N II]/H-Alpha ratios increase as absolute H-Alpha intensitiesdecrease. For the more distant Perseus arm emission, the increase in theseratios is a strong function of Galactic latitude and thus, of height above theGalactic plane. The [S II]/[N II] ratio is relatively independent of H-Alphaintensity. Scatter in this ratio appears to be physically significant, and mapsof it suggest regions with similar ratios are spatially correlated. The Perseusarm [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 H-Alpha intensities,the increase of [S II]/H-Alpha and [N II]/H-Alpha with |z| seems to reflect anincrease in temperature. Such an interpretation allows us to estimate thetemperature and ionization conditions in our large sample of observations. Wefind that WIM temperatures range from 6,000 K to 9,000 K with temperatureincreasing from bright to faint H-Alpha emission (low to high [S II]/H-Alphaand [N II]/H-Alpha) respectively. Changes in [S II]/[N II] appear to reflectchanges in the local ionization conditions (e.g. the S+/S++ ratio). We alsomeasure the electron scale height in the Perseus arm to be 1.0+/-0.1 kpc,confirming earlier, less accurate determinations.