This picture of airglow ripples over Tibet was made Astronomy Picture of the Day on September 1, 2014. After a giant thunderstorm in Bangladesh, these ripples of glowing air appeared over Tibet. The pattern is created by gravity waves, which are waves of alternating pressure that grow with height as the air thins. These particular gravity waves existed 90 kilometers in the sky. Although this phenomenon appears to be an aurora, it is actually not- whereas auroras are powered by collisions of charged particles at high altitudes, the airglow ripple effect is caused by chemiluminescence, the production of light in a chemical reaction. Airglow is more typically seen on the horizon, making this a particularly odd phenomenon. Airglow actually keeps the night sky from ever being completely dark.
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