This article discusses the colors of the aurora borealis and the physical processes that cause the different colors. It begins with a brief history of observations and understanding of the aurora. It then explains that the colors are caused by atoms and molecules in the upper atmosphere emitting light as they are struck by energetic particles from space. Specifically, green light is emitted by oxygen atoms, while red and purple colors come from higher altitude emissions. By analyzing the color balance and structure of the aurora, scientists can learn about the atmospheric composition and electric currents in near-Earth space that generate the colorful northern lights.

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2. Alaska Park Science, Volume 8, Issue 1
The Colors of the Aurora
By Dirk Lummerzheim for his duties as a ruler. reflected by ice crystals in the air, the glow
The term aurora borealis originated of glacier ice near the pole, or a light
Abstract in the 1600s, when Galileo combined the emanating from the edge of Earth. In the
The aurora has fascinated observers word “aurora,” the Latin word for “dawn,” eighteenth century, scientists discovered
at high latitudes for centuries, but only with the term “boreal,” the Greek word a connection between the aurora and
recently have we begun to understand the for “north.” Aurora also appears in the disturbances in Earth’s magnetic field
processes that cause it. This article discuss- southern hemisphere, where it is called and associated aurora with sunspots.
es the mechanisms that are responsible for the “aurora australis.” Since there is very But it took until the end of the twentieth
the colors of the aurora. Observations of little populated landmass at high southern century before a satisfactory explanation of
color balance in aurora can provide us with latitudes, there are no known historical and the aurora, its colors, and the mechanisms
information about the physical processes mythological references to the southern behind it emerged.
in the near Earth space that cause aurora. aurora. Although the native people from
High-resolution spectral observations let New Zealand must have seen aurora on The Processes that Cause Aurora
us understand how the upper atmosphere occasion, Captain Cook is considered the The light of the aurora is generated by
is affected by aurora. discoverer of the aurora australis; he saw it atoms and molecules of the air when they
in 1773 on his voyage around the southern are struck with energetic particles from
A Brief History of Understanding tip of South America. space. These energetic particles come from
the Aurora At mid-latitudes, people rarely see the the volume of space just above the aurora,
Descriptions of aurora, or the northern lights. Aurora is visible at mid- and are accelerated by plasma physics
northern lights, go as far back as latitudes during the largest magnet- processes that are still under investi-
written history. 2,300 years ago, Aristotle ic storms, but it is dominated by red gation. But we do have a fairly good
saw curtains of light in the sky and called colors. In ancient times when the understanding of the general processes
the phenomena “chasmata” to indicate that aurora appeared overhead, people often and the flow of energy that feeds these
the cause was cracks in the sky, allowing associated the aurora with good or bad processes. We can model the aurora and
in light from beyond the heavenly sphere. omens and sometimes considered it a are now gaining the understanding to
In his book Majestic Lights, Eather (1980) manifestation of activities of heavenly forecast its appearance (Lummerzheim
presents several quotes from the Bible that spirits or gods. The peoples who lived at 2007).
most likely refer to aurora. high latitude and who had a regular display Charged particles, like those that cause
The first recorded use of the words of the aurora held similar beliefs. the aurora, can generally only travel along
“northern lights” to describe the aurora In the Middle Ages, scientists came the direction of the magnetic field. This
Figure 1. Multi-colored aurora over Klondike
Gold Rush National Historical Park near
was in 1230, in a book titled The King’s up with other guesses as to what was shapes the aurora into curtain and ray-
White Pass in Southeast Alaska. Mirror. The author wrote the book to pre- behind the northern lights: they suggested like structures (Figure 2). Following the
Photograph courtesy of Michael Klensch pare Norwegian King Magnus Lagabøte that the light of the aurora was sunlight magnetic field up from the aurora, we
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3. The Colors of the Aurora
get to the auroral acceleration region,
Photograph courtesy of D. Lummerzheim
Photograph courtesy of Michael Klensch
about 620-6,200 miles (1,000-10,000
km) above the earth. The smaller scale
structures like rays, small curls, and thin
curtains shown in Figure 3 represent
structures in the acceleration processes.
This region is connected to the outer
magnetosphere by electric currents. Large-
scale structures, like multiple parallel arcs
Figure 2. Green curtains and rays above (Figures 4-5) and spirals that fill almost
the Brooks Range and Gates of the Arctic
National Park.
the entire sky (Figure 6) show the spatial
pattern of these currents. The magneto-
sphere is the region of space around Earth
Photograph courtesy of Poul Jensen
that is controlled by Earth’s magnetic field.
Its diameter is about 30 Earth radii, and out-
side of the magnetosphere is the solar wind.
The magnetosphere forms an obstacle for
the solar wind, which has to flow around it.
This interaction of the solar wind with the
magnetosphere provides the energy that
eventually accelerates the auroral electrons
Figure 3. Small-scale structure in aurora in the inner magnetosphere. Strong solar
shows as thin curtains and small rays and activity causes strong variations in the solar
curls over Fairbanks, Alaska. This structure wind; byproducts of this space weather are
is related to the auroral acceleration process
directly above the atmosphere. therefore geomagnetic storms and aurora.
Light Emission in Aurora
Photograph courtesy of Carl Johnson
When energetic electrons strike an
atom or molecule, they slow down and
transfer some of their energy to that atom
or molecule. The molecules can store this
energy only for a very short time, and then
radiate the energy away as light. Some
molecules get dissociated into atoms in
this process, and some molecules and
Figure 4. Large-scale structure in aurora atoms get ionized. At the altitude where
shows as large folds and parallel curtains aurora occurs, above about 62 miles (100
over Gates of the Arctic National Park and
Preserve. This structure reflects the pro-
km), the air is thin enough that oxygen can
cesses in the magnetosphere where large exist in atomic form, while the air that we
currents transport energy into the auroral Figure 5. Several parallel curtains above Rock Creek in Denali National Park and Preserve. breathe contains only molecular oxygen.
region. Note comet Hale-Bopp in the lower right hand corner.
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4. Alaska Park Science, Volume 8, Issue 1
During the day, the ultraviolet sunlight 0.7-second lifetime allows radiation. This auroral brightness depends on the rate of
Photograph courtesy of Poul Jensen
splits the molecular oxygen into atoms, determines the bottom edge of the green incoming auroral electrons. In analogy to
while at night the aurora continues this emission in aurora. electric currents we can say that the color
process. However, the auroral electrons some- tells us the voltage, the brightness tells us
When an atom or molecule emits times have enough energy to give them the the current.
light as a photon, to rid itself of its excess punch to penetrate deeper than that into During very large magnetic storms
energy, that photon has a wavelength the atmosphere. When that happens, only the aurora is visible from mid and low
that is characteristic for that atom. We emissions with a much shorter lifetime latitudes. It is common to have very bright
perceive wavelength as color. Laboratory are possible. The most abundant gas is red auroral displays during such large
experiments can reproduce these light- molecular nitrogen, and it radiates storms. Notable were the magnetic storms Figure 6. A large spiral that fills a large por-
tion of the sky in this extreme wide-angle
emitting processes by forcing a current promptly in deep blue and red colors. on Halloween and late November in 2003, photo from Ester Dome near Fairbanks,
through an evacuated glass tube that Mixing these together gives purple. The when red aurora was seen above the Medi- Alaska.
contains a small amount of a selected gas. bottom edge of a green auroral curtain terranean, Florida, and the entire U.S. The
Photograph courtesy of Poul Jensen
The study of these light-emitting processes gets this purple color when auroral elec- outstanding brightness and dominance of
led to the understanding of atoms early in trons are accelerated to very high energy high altitude red oxygen emissions indi-
the twentieth century, and to the discovery (Figures 7-8). cate that during such storms the magne-
of quantum mechanics. Because each type On occasion the aurora gets a deep red tosphere has very large currents flowing,
of atom or molecule emits colors unique color. This comes from higher altitudes, while the auroral acceleration only pro-
to it, we can use the colors of the aurora to around 120-180 miles (200-300 km). It is duces low energy electrons.
determine the atmospheric composition at again the oxygen atom that is responsible In addition to looking at the color
the auroral altitude. for this color. The oxygen atom has an balance and brightness, we can measure
The time that a molecule or atom can excited state for this red line emission the wavelength of individual emission lines
Figure 7. Intense aurora develops a purple
store the energy that it gained in a colli- with a mean lifetime of 100 seconds, and in the aurora with very high accuracy. This border below the green curtains in this fish
sion is very short, typically between 1/1000 only at very high altitudes are collisions allows us to determine the Doppler shift eye view of almost the entire sky above
and less than 1/1,000,000 of a second. infrequent enough to allow this radiation of emission lines. The Doppler effect for Fairbanks, Alaska. Note the Big Dipper near
the zenith.
Atomic oxygen is one notable exception, to be emitted (Figure 9). Since the long life- light emission causes a shortening of the
and the excited state that causes the most time of the oxygen red line also allows the wavelength of the emission if the emitting
Photograph courtesy of Poul Jensen
common auroral emission, the green line, aurora to move before it radiates, the de- atom or molecule is moving toward the
has a lifetime of 0.7 seconds. When an tailed structure in auroral curtains is also observer, and a lengthening of the wave-
excited atom takes that long to radiate washed out in these emissions (Figure 10). length if it is moving away. A shorter wave-
away the internally stored energy, other length means a color closer to the blue
processes, chemical reactions or collisions, Relating the Color to Physical end of the spectrum; longer wavelength
compete with the radiation process for Processes means a shift to red. In aurora, these shifts
that energy. The denser the air is, the more The energy of auroral electrons are miniscule, but can be observed with
frequent are the collisions between the determines how deep into the atmosphere high spectral resolution instruments, in
atoms and molecules. Below the altitude these particles penetrate. Since auroral particular Fabry-Perot interferometers Figure 8. High-energy auroral electrons
of about 59 miles (95 km), collisions are so emissions are characteristic of the altitude (FPI). Because the red and green line above Fairbanks, Alaska penetrate deep
enough to cause the purple lower border of
frequent that the green oxygen line has no where they originate, we can use the color emissions from atomic oxygen are so long the green curtains. High-energy aurora also
chance to be emitted. All the energy that is balance of the aurora to determine the lived, they are good candidates for FPI produces highly structured and very thin
put into the oxygen atom is lost before the energy of the auroral electrons. The observations. The long lifetime ensures curtains that move fast. Short exposure times
are necessary to resolve these structures.
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5. The Colors of the Aurora
Photograph courtesy of Michael Klensch
Photograph courtesy of Michael Klensch
Figure 10. The diffuse looking red aurora
above Klondike Gold Rush National Historical
Park near White Pass comes from long-lived
oxygen atoms at high altitudes. The structure
of the curtain below is lost in the red aurora
because the excited atoms can move with
the wind before emitting light.
Image from Conde et al. 2008
Figure 9. Red aurora above the Sawtooth Mountains near Klondike Gold Rush National Historical Park and Skagway, Alaska Figure 11. The wind vectors at 150 miles (240
km) altitude drawn over a composite all-sky
image of the green and red oxygen emission.
Through its interaction with the atmosphere,
the aurora modifies the wind direction and
speed in the upper atmosphere.
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6. Alaska Park Science, Volume 8, Issue 1
us much about the physical processes that
Photograph courtesy of Michael Klensch
Photograph courtesy of Jan Curtis
cause aurora and the effects that aurora
has on the upper atmosphere. The color
balance tells us the altitude of aurora.
We can relate that to the processes that
accelerate auroral electrons in the
near earth space, and we can see the
evolution of electric currents that flow
in the magnetosphere. High-resolution
spectroscopy lets us see the wind in the
upper atmosphere and how it is changed
by the aurora. High altitude blue aurora
tells us that ions that are generated in the
aurora are pulled out of the atmosphere
into space. And the colors themselves tell
us the composition of the atmospheric gas
at the altitude of the aurora.
Figure 12. Aurora over Klondike Gold Rush National Historical Park as seen from Yukon,
Canada.
Acknowledgements
that the atoms are drifting with the wind, The top end of the auroral curtains Figure 13. This photo is taken shortly after
I would like to thank Mark Conde sunset, when the sun illuminates the upper
and their velocity is not affected by the and rays sometimes show a deep blue
for suggestions and comments, and the part of the aurora. Ions that are produced
collision with the auroral electron that color. This is indicative of still another light by the aurora at these altitudes scatter the
people who have provided the photo-
caused the excitation in the first place. The emission process. The auroral electrons blue part of the sunlight, causing the upper
graphs of the aurora for this article: Poul edge of the aurora to look blue.
small shift in wavelength can thus be used not only produce light emitting excited
Jensen at the Geophysical Institute at
to measure the ambient wind at the alti- atoms and molecules, they also ionize
the University of Alaska Fairbanks; Mi-
tude of the aurora. some molecules. These ions can then be
chael Klensch from Skagway, Alaska; Carl
These FPI wind observations only give pulled upward by electric fields in the REFERENCES
Johnson from Anchorage, Alaska; and
the component of the wind velocity along aurora and reach altitudes high enough that
Jan Curtis from Oregon. More aurora Conde, M.G., C. Anderson, and C. Ander-
the line of sight, the component toward under some conditions they will be son. 2008.
photos by these photographers can be
or away from the observing station. By exposed to sunlight. This sunlight then New results from the Poker Flat all-sky
found at their websites:
using model constraints or by placing scatters off these ions. There exists a blue imaging Fabry-Perot spectrometer.
three such instruments in separate loca- emission of the molecular nitrogen ion that www.gfy.ku.dk/~flyvholm (P. Jensen) CEDAR workshop, Zermatt, UT.
tions we can reconstruct the actual wind is particularly strong in scattering sunlight, www.muk.uni-hannover.de/~theusner/
Eather, R.H. 1980.
vector by measuring three components of it. which is why we see a blue upper end of polarlicht/ (M. Theusner) Majestic Lights: The Aurora in Science,
Figure 11 shows a composite of the auroral the auroral curtains (Figure 13). www.alpenglowphoto.net (M. Klensch) History, and the Arts. American Geo-
brightness with the deduced wind physical Union. Washington DC.
climate.gi.alaska.edu/Curtis/aurora/
vectors at 150 miles (240 km) altitude Putting it all Together
aurora.html (J. Curtis) Lummerzheim, D. 2007.
superimposed. This example (Conde et al. The observations of the colors of
www.carljohnsonphoto.com (C. Johnson) Modeling and forecasting aurora.
2008) shows that the wind is affected by the the aurora, either in a broader sense by
Computing in Science and Engineering
aurora as the direction and speed changes looking at the overall color balance, or by
9(5):53-61.
right at the position of the auroral curtain. detailed spectroscopic methods, can teach
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