WHAM OBSERVATIONS OF Ha, [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¡È164¡, 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 Ha Mapper (WHAM) in the [S II] j6716 and [N II] j6583 lines. By comparing these data with the maps from the WHAM Ha 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 diuse gas in other galaxies are conÐrmed in the Milky Way and extended to much fainter emission. We Ðnd that the [S II]/Ha and [N II]/Ha ratios increase as absolute Ha 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 Ha intensity. Scatter in this ratio appears to be physically signiÐcant, and maps of [S II]/[N II] suggest that regions with similar ratios are spatially correlated. The Perseus arm [S II]/[N II] ratio is systematically lower than local emission by 10%È20%. With [S II]/[N II] fairly constant over a large range of Ha intensities, the increase of [S II]/Ha and [N II]/Ha with o z o seems to reÑect an increase in temperature. Such an inter-pretation allows us to estimate the temperature and ionization conditions in our large sample of obser