Colored by the atmosphere

Different gases give off different colors of light when excited. The colors of the aurora come from oxygen and nitrogen gas.

 

Oxygen
Oxygen atoms produce both green light and a brownish red at the limits of human perception.

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Different gases, colors
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Nitrogen
Nitrogen molecules are energized in two different ways to produce two different colors.

  • Ionized nitrogen
    When high energy electrons strike nitrogen molecules, some of the nitrogen's own electrons are knocked loose. When nitrogen regains its lost electrons, returning to its normal energetic state, the extra energy is released as bluish light.
  • Excited nitrogen
    Nitrogen is excited in the same was as oxygen. It absorbs energy from high energy electrons and releases the energy as red light.
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Solar vs. auroral spectrum
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Colors and altitude
The aurora is made up of blue, green, and red light. The highest part of the auroral curtain is red, the middle is greenish-white and the lower edge is pink. These color variances are due to the nature of the atmosphere at these different altitudes and the way oxygen emits light. Most atoms and molecules release the extra energy and emit light microseconds after being excited. Oxygen, however, takes nearly a second to emit energy as green light and up to two minutes to emit red light. If the excited oxygen should collide with another molecule, it may simply pass the extra energy on to the other molecule without emitting light.

The atmosphere at high altitudes contains a greater percentage of atomic oxygen and is very thin giving the atoms ample opportunity to emit red light. At more moderate altitudes, the combination of red, blue, and green lights from oxygen and nitrogen combine to produce greenish-white light. At the lower edge of the curtain, the density of molecules doesn't permit oxygen to emit light; the pinkish color comes from a combination of red and blue from nitrogen.

 


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Anders Jonas Ångstrom
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Auroral light is not sunlight
Seen through a prism, the sun's spectrum is the familiar rainbow: a continuous transition from red to violet. The spectrum of auroral light, on the other hand, forms distinct lines and bands of color. These lines and bands are discrete wavelengths of light characteristic of the atoms and molecules that produce them.

Anders Jonas Ångstrom (1814-1874) was one of the first to discover the auroral spectrum and note its distinct differences from the sun's spectrum.

 



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site last modified: August 2003 maintained by Asahi Aurora Web Manager