Atmospheric Phenomena

Atmosphere's Own Light

Apart from lightning, the atmosphere can make at least two other sorts of light, seen only at night.

Airglow is weak light emitted by the upper atmosphere from chemical processes and from interactions between air molecules and cosmic rays. It is dim, about 0.3 ten-thousandths of a candle per square meter, visible only to rod vision as a slight colorless glow. It means, however, that even the darkest sky between the stars is not completely dark.

Aurorae are much stronger lights emitted by molecules of the upper atmosphere when bombarded by energetic particles from the sun. The particles follow the earth's magnetic field lines down the atmosphere, exciting oxygen molecules, giving red and green light, and nitrogen molecules, giving blue-violet light. Aurorae form in a ring between about 20 and 30° latitude from the magnetic poles.

Aurorae can be dim, visible only to rod vision, hence colorless. But other times, they can be bright enough to be seen in color by cone vision above other light sources such as the moon and streetlights. Aurorae can appear as diffuse glows or, startlingly, as vertical rays that look like curtains running approximately east-west. They can be essentially stationary or can form and reform over tens of seconds. The curtains can appear to be blowing about as if in a wind, parts overlapping with other parts. All this light and change occurs completely silently. Aurorae are some of the biggest things one can see in the atmosphere, being 80 kilometers (km) high and thousands of kilometers long. Occasionally the rays can descend directly over an observer, appearing as pattern of diverging rays from linear perspective.

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