Aurora Borealis
What Causes the Aurora Borealis Colors?
Aurora Borealis
Color Science
The aurora is the name given
to the bands of colored lights seen in the sky at the higher latitudes.
The aurora borealis or
Northern Lights are seen mainly near the Arctic Circle.
The aurora australis or
Southern Lights are seen in the southern hemisphere.
The light you see comes from
photons released by oxygen and nitrogen in the upper atmosphere.
Energetic particles from the
solar wind strike the layer of the atmosphere called the ionosphere,
ionizing the atoms and molecules.
When
the ions return to the ground state, energy released as light produces the
aurora.
Each
element releases specific wavelengths, so the colors you see depend on the type
of atom that is excited, how much energy it received, and how the wavelengths
of light blend with each other.
Scattered
light from the sun and moon may affect the colors, too.
Aurora Colored - From Top to Bottom
You
can see a solid-colored aurora, but it’s possible to get a rainbow-like effect
through the bands.
Scattered light from the sun
can impart a violet or purple to the top of an aurora.
Next, there may be red light
atop a green or yellow-green band.
There may be blue with the
green or below it. The base of the aurora may be pink.
Solid Colored Aurora
Solid
green and solid red auroras have been seen.
Green is common at the upper
latitudes, while red is rare.
On the other hand, aurora
viewed from the lower latitudes tend to be red.
Element Emission Colors
Oxygen
The
big player in the aurora is oxygen.
Oxygen is responsible for the
vivid green (wavelength of 557.7 nm) and also for a deep brownish red
(wavelength of 630.0 nm).
Pure green and greenish
yellow aurorae result from excitation of oxygen.
Nitrogen
Nitrogen emits blue (multiple
wavelength) and red light.
Other Gases
Other
gases in the atmosphere become excited and emit light, although the wavelengths
may be outside of the range of human vision or else too faint to see.
Hydrogen and helium, for
example, emit blue and purple.
Although our eyes can't see
all of these colors, photographic film and digital cameras often record a wider
range of hues.
Aurora Colors According to Altitude
above 150 miles -- red
-- oxygen
up to 150 miles -- green -- oxygen
above 60 miles -- purple or violet -- nitrogen
up to 60 miles -- blue -- nitrogen
up to 150 miles -- green -- oxygen
above 60 miles -- purple or violet -- nitrogen
up to 60 miles -- blue -- nitrogen
Black Aurora?
Sometimes
there are black bands in an aurora.
The black region can have
structure and block out starlight, so they appear to have substance.
The black aurora most likely
results from electric fields in the upper atmosphere that prevent electrons
from interacting with gases.
Aurora on other Planets
The
Earth is not the only planet that has aurorae.
Astronomers have photographed
the aurora on Jupiter, Saturn, and Io, for example.
However, the colors of the
aurora are different on different worlds because the atmosphere is different.
The only requirement for a
planet or moon to have an aurora is that it have an atmosphere that is
bombarded by energetic particles.
The
aurora will have an oval shape at both poles if the planet has a magnetic
field.
Planets
without magnetic fields still have an aurora, but it will be irregularly
shaped.
Anne
Marie Helmenstine, Ph.D.
Anne Helmenstine, Ph.D., is an author and
consultant with a broad scientific and medical background.
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