What is the color of space?
Stan Solomon captured this image
of the aurora above the Mesa Lab during the aftermath
of a large geomagnetic storm on November 20, 2003.
It may look black, but the correct answer is that space
is predominantly green. And sometimes red, blue, and purple
During an ESSL lecture on April 11, HAO’s Stan Solomon
broke down the physics of the aurora borealis in an informative,
entertaining fashion for a crowd of staff from across UCAR/NCAR/UOP.
Drawing from sources as diverse as The Onion and medieval
European art, he addressed basic questions about the aurora:
Where and when can one see it? What causes it? What color
is it, and why does it sometimes have different colors?
The aurora borealis (called aurora australis in the Southern
Hemisphere) intensifies following coronal mass ejections—that
is, explosions in the Sun’s corona that spew particles
into the solar wind. When the charged electrons and protons
encounter Earth’s magnetosphere (the area of space
around Earth that is controlled by Earth’s magnetic
field), they travel along Earth’s magnetic field lines
to the polar regions, colliding with atoms and molecules
in the upper atmosphere. The atoms and molecules become electronically
excited from the collisions. As they relax into their normal
state, we see this release of energy in the form of colored
The color of the aurora depends upon the composition of Earth’s
atmosphere at the altitude of the aurora, since different
gases at varying levels in the atmosphere give off distinct
colors when they are excited. Oxygen atoms about 100 kilometers
(60 miles) high produce the vivid green light for which the
aurora is best known, while oxygen around 200 km (120 mi) high emits
a red glow.
Nitrogen molecules produce blues
“The predominant experience is mostly green,” Stan
When a magnetic field line loops downward directly above
an observer, the visual effect of rays beaming out in all
directions is called an aurora corona. “It’s
almost overwhelming to be underneath an aurora corona with
it whirling around,” Stan said.
Stan’s lecture was the first in an ESSL series that
aims to provide scientists, nonscientists, and the general
public with accessible, intriguing presentations about the
lab’s research. Upcoming lectures will cover climate
change, hurricanes, and ozone.
In this issue...
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is the color of space?
Just One Look
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