Spatial variations of the [ S II ] /H \alpha and [ N II ] /H \alpha line intensity ratios observed in the gaseous halo of the Milky Way and other galaxies are inconsistent with pure photoionization models .
They appear to require a supplemental heating mechanism that increases the electron temperature at low densities n _ { e } .
This would imply that in addition to photoionization , which has a heating rate per unit volume proportional to n _ { e } ^ { 2 } , there is another source of heat with a rate per unit volume proportional to a lower power of n _ { e } .
One possible mechanism is the dissipation of interstellar plasma turbulence , which according to Minter & Spangler ( 1997 ) heats the ionized interstellar medium in the Milky Way at a rate \sim 1 \times 10 ^ { -25 } n _ { e } ergs cm ^ { -3 } s ^ { -1 } .
If such a source were present , it would dominate over photoionization heating in regions where n _ { e } \lesssim 0.1 cm ^ { -3 } , producing the observed increases in the [ S II ] /H \alpha and [ N II ] /H \alpha intensity ratios at large distances from the galactic midplane , as well as accounting for the constancy of [ S II ] / [ N II ] , which is not explained by pure photoionization .
Other supplemental heating sources , such as magnetic reconnection , cosmic rays , or photoelectric emission from small grains , could also account for these observations , provided they supply to the warm ionized medium \sim 10 ^ { -5 } ergs s ^ { -1 } per cm ^ { 2 } of Galactic disk .