Just back from another weekend adventure out to the dark Skies of Portal, AZ. Seems that my blog is slowly turning into the Portal report...in any event, below is an image of the Milky Way just after it cleared sufficiently above the horizon for a picture. In this image we are looking toward the center of our galaxy, in the constellation of Sagittarius.
The green glow just above the horizon is air glow- light being emitted by Earth's atmosphere. This is from several sources, such as the recombination of atoms which were ionized by the sun during the day, light caused by cosmic rays striking the upper atmosphere, and energy emitted by the combining of nitrogen and oxygen in the upper atmosphere.
While the entire sky has air glow (and as a result it is never truly dark), the glow is most obvious about 10-20 degrees above the horizon. This is because the glow is subtle, and we are looking through about twice as much atmosphere looking across the horizon than when peering overhead. (We do not notice the air glow on the horizon due to atmospheric extinction of the light on the horizon).