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 atoms produce both green light and a brownish red at the limits of human perception.
Different gases, colors
click to see larger image
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.
Solar vs. auroral spectrum
click to see larger image
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.
Anders Jonas Ångstrom
click to see larger image
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.
Geophysical Institute
903 Koyukuk Drive, University of Alaska Fairbanks, Fairbanks, Alaska 99775-7320
site last modified: August 2003 maintained by Asahi Aurora
Web Manager