Excited atoms produce light

Auroral light occurs when atoms and molecules of gases in the upper atmosphere are struck by high-energy electrons.

Gases of the atmosphere
Earth's atmosphere is 78% nitrogen and 21% oxygen. All other gases make up the remaining 1%. The atmosphere is thickest close to the surface and grows thinner with increasing altitude.

Upper atmosphere
The aurora has a curtain-like shape and its lower edge is over 60 miles (100 km) above Earth's surface. The altitude of the upper edge can be over 300 miles (483 km) high. The aurora is not a weather phenomenon, almost all weather occurs in the first 10 miles (16 km) of the atmosphere.

Collisions create light
When a high-energy electron collides with an oxygen atom, a small amount of the electron's energy is transferred to the oxygen's electrons. The oxygen is in an "excited state" when it is energized in this way. The oxygen's electrons hold this extra energy briefly, then releases it as light. Large numbers of excited atoms produce light bright enough to be visible.

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Oxygen is not the only gas that gives off light. Other gases including nitrogen and hydrogen also produce light when struck by high-energy electrons. Neon signs harness this phenomenon. Electricity passing through neon gas produces their familiar red glow.

Geophysical Institute
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site last modified: August 2003 maintained by Asahi Aurora Web Author