Wham observations of H alpha, [S II], and [N II] toward the Orion and Perseus arms: Probing the physical conditions of the warm ionized medium

A large portion of the Galaxy (l = 123 degrees-164 degrees, b = -6 degrees to -35 degrees), which samples regions of the Local (Orion) spiral arm and the more distant Perseus arm, has been mapped with the Wisconsin H alpha Ma pper (WHAM) in the [S II] lambda 6716 and [N II] lambda 6583 lines. By comp aring these data with the maps from the WHAM Ha Sky Survey, we begin an inv estigation of the global physical properties of the warm ionized medium (WI M) in the Galaxy. Several trends noticed in emission-line investigations of diffuse gas in other galaxies are con firmed in the Milky Way and extended to much fainter emission. We find that the [S II]/H alpha and [N II]/H alp ha ratios increase as absolute H alpha intensities decrease. For the more d istant Perseus arm emission, the increase in these ratios is a strong funct ion of Galactic latitude, b, and thus of height, z, above the Galactic plan e, while the [S II]/[N II] ratio is relatively independent of H alpha inten sity. Scatter in this ratio appears to be physically significant, and maps of [S II]/[N II] suggest that regions with similar ratios are spatially cor related. The Perseus arm [S II]/[N II] ratio is systematically lower than l ocal emission by 10%-20%. With [S II]/[N II] fairly constant over a large r ange of H alpha intensities, the increase of [S II]/H alpha and [N II]/H al pha with /z/ seems to reflect an increase in temperature. Such an interpret ation allows us to estimate the temperature and ionization conditions in ou r large sample of observations. We find that WIM temperatures range from 60 00 to 10,000 K, with temperature increasing from bright to faint H alpha em ission (low to high [S II]/H alpha and [N II]/H alpha), respectively. Chang es in [S II]/[N II] appear to reflect changes in the local ionization condi tions (i.e., the S+/S++ ratio). We also measure the electron scale height i n the Perseus arm to be 1.0 +/- 0.1 kpc, confirming earlier, less accurate determinations.