The altitude variation of the Atmosphere Explorer optical glow intensity suggests that two different processes are responsible for the glow. One, dominant for altitudes above 180 km, has an emission brightness proportional to the ambient atomic oxygen density whereas the other, dominant at altitudes below 160 km, produces an emission whose intensity is proportional to the product of the densities of any of N2, O2 or NO. The first mechanism apparently has two components, one from the surface recombination of O and H and the other from a process similar to that producing the Shuttle glow. Unless the efficiency of the second mechanism is much enhanced on the Shuttle its contribution to the Shuttle glow is negligible.