May 2007

What is the color of space?

aurora

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 as well.

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 light.

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 and purples.

“The predominant experience is mostly green,” Stan said.

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.