Mysterious Green 'Airglow' Spotted Hanging Just Over Earth

There are three categories of airglow including dayglow, nightglow, and twilightglow, each being the outcome of the interaction between sunlight and the molecules floating in the atmosphere.

How do airglows form?

Dayglow is formed when sunlight collides with the atmospheric molecules during the daytime. When that happens, some of the Sunlight is absorbed by the molecules, acquiring excess energy in the process. The molecules then release this energy as visible light. Because the light is much dimmer during the daytime, we can't spot it with naked eyes.

Twilight glow forms basically under the same circumstances as dayglow, but only the upper atmosphere is lit in this case. Because the lower parts of the atmosphere and the observer are in darkness, twilightglow is visible to the naked eyes.

The chemistry that leads up to nightglow is somewhat different by nature. In this case, there's no sunlight interacting with the nighttime atmosphere. It is rather caused by a different process called chemiluminescence. Here's in brief how chemiluminescence works:

As the molecules in the atmosphere gather energy from sunlight during the day, the oxygen molecules at an altitude of nearly 100km get ripped apart by that energy. However, unable to get rid of the excess energy that easily, the individual oxygen atoms store it for several hours.

But after a certain cooldown period, the oxygen atoms recombine to form molecular oxygen again. This molecular oxygen then releases the energy in the form of light, causing nightglow.