2014ApJ...787..106H

We present Wisconsin H-Alpha Mapper [SII] λ6716 and Hα spectroscopic maps of the warm ionized medium (WIM) in the Scutum-Centaurus Arm at Galactic longitudes 310° < l < 345°. Using extinction-corrected Hα intensities (I_Hα^c), we measure an exponential scale height of electron density squared in the arm of H_ne2^_ = 0.30kpc (assuming a distance of 3.5 kpc), intermediate between that observed in the inner Galaxy and in the Perseus Arm. The [S II]/Hα line ratio is enhanced at large|z| and in sightlines with faint (I_Hα^c. We find that the [S II]/Hα line ratio has a power-law relationship with (I_Hα^c from a value of ~1.0 at (I_Hα^c < 0.2R (Rayleighs) to a value of ~0.08 at (I_Hα^c ≳ 100R. The line ratio is better correlated with Hα intensity than with height above the plane, indicating that the physical conditions within the WIM vary systematically with electron density. We argue that the variation of the line ratio with height is a consequence of the decrease of electron density with height. Our results reinforce the well-established picture in which the diffuse Hα emission is due primarily to emission from in situ photoionized gas, with scattered light only a minor contributor.