A large portion of the Galaxy ( \ell = 123 \arcdeg to 164 \arcdeg , b = -6 \arcdeg to -35 \arcdeg ) , 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 [ \ion S2 ] \lambda 6716 and [ \ion N2 ] \lambda 6583 lines .
By comparing these data with the maps from the WHAM H \alpha 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 [ \ion S2 ] /H \alpha and [ \ion N2 ] /H \alpha ratios increase as absolute H \alpha 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 [ \ion S2 ] / [ \ion N2 ] ratio is relatively independent of H \alpha intensity .
Scatter in this ratio appears to be physically significant , and maps of [ \ion S2 ] / [ \ion N2 ] suggest regions with similar ratios are spatially correlated .
The Perseus arm [ \ion S2 ] / [ \ion N2 ] ratio is systematically lower than Local emission by 10 % –20 % .
With [ \ion S2 ] / [ \ion N2 ] fairly constant over a large range of H \alpha intensities , the increase of [ \ion S2 ] /H \alpha and [ \ion N2 ] /H \alpha 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 H \alpha emission ( low to high [ \ion S2 ] /H \alpha and [ \ion N2 ] /H \alpha ) respectively .
Changes in [ \ion S2 ] / [ \ion N2 ] 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 \pm 0.1 kpc , confirming earlier , less accurate determinations .