Air Glow Over Kitt Peak

Another shot from my night on Kitt Peak a few days ago. This is a wide shot taken from the Visitor's Center. 

You can see the iconic McMath-Pierce Solar Telescope in the background and the sky glow from Tucson on the left. However, if you look on the right side of the image, you will notice a faint green glow and possibly even fainter red if you look close. This is a real phenomena called air glow. Due to air glow, the sky is never completely dark. Airglow occurs all over the sky but is usually more pronounced close to the horizon since you are looking through a thicker layer of atmosphere.

There are several sources of air glow. During the day, atoms in the upper atmosphere are ionized by sunlight. When the ionized atoms and electrons recombine, they give off light...and oxygen can give off green light in the process. Cosmic rays striking the upper atmosphere contribute to air glow as well. Finally, oxygen and nitrogen can react with hydroxyl ions in the upper atmosphere and give off light through a process called chemiluminescence. 

Air glow is not visible during the day as it far too faint. Even at night, you need to be in a very dark site to capture it. Thursday night was the first time I have captured this elusive phenomena.

Reprinted with permission from the Half-Astrophysicist Blog.