1999ApJ...525L..21R

Spatial variations of the [S II]/Hα and [N II]/Hα 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 n²_e, 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, heats the ionized interstellar medium in the Milky Way at a rate of ∼1x10^–25n_e ergs.cm^–3.s^–1. If such a source were present, it would dominate over photoionization heating in regions where n_e≲0.1 cm^–3, producing the observed increases in the [S II]/Hα and [N II]/Hα 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 ∼10^–5 ergs.s^–1 per square centimeter of the Galactic disk to the warm ionized medium.