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Why can auroras appear green, red, or white?
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Ionosphere and magnetosphere
The layers of Earth's atmosphere, with a yellow line showing the air temperature at various heights.
Auroras are luminous atmospheric phenomena caused by the collision of high-energy charged particles from the solar wind with atoms in Earth's upper atmosphere. As these particles travel along magnetic field lines toward the poles, they strike oxygen and nitrogen atoms, leaving them in excited states. When these atoms return to their normal state, they emit light at various wavelengths, creating the characteristic colors of the aurora.312
The specific color observed depends on which atmospheric species is primarily emitting light:1
- Green: Produced by collisions with atomic oxygen.1
- Red: Also produced by collisions with atomic oxygen, typically at different altitudes.31
- White: Often appears as the rays of an aurora fade into diffuse areas of light.2
- Purple: Produced by collisions with nitrogen.3
These emissions generally occur at altitudes between 80 and 250 km (about 50 to 155 miles) above Earth's surface, with the most common displays appearing at approximately 100 km (60 miles).312