Shop Windows to the Universe

Young Voices for the Planet DVD in our online store includes 8 films where students speak out and take action on climate change.

Charged Particle Motion in Earth's Magnetosphere

Auroral Colors and Spectra

When the aurora is faint, it appears white to the unaided eye. As its brightness increases, color vision starts to work, and the characteristic pale green hue of the aurora becomes visible.

Green aurora

With more intensity, a bright green color is visible in the lower regions, and a faint red glow can be discerned at high altitude.

Green and red aurora

When the aurora is very energetic and very bright, a deep red lower border appears below the green.

Green aurora with deep red lower fringe

The atmosphere above 100 kilometers consists mainly of nitrogen molecules and oxygen atoms, with more nitrogen at 100 kilometers transitioning to proportionally more oxygen at 200 kilometers.

Graph of oxygen and nitrogen variation in the thermosphere

When oxygen atoms are excited, they emit light of various colors, but the brightest are the red and green emissions.

Atomic oxygen emission spectrum

Excited nitrogen molecules emit red, blue, and violet light. The interplay of oxygen and nitrogen and their proportional changes with altitude cause the typical appearance of auroral colors.

Molecular nitrogen emission spectrum

Some of the excited nitrogen molecules also interact with the atomic oxygen, causing an additional green emission at a wavelength of 558 nanometers. This emission, called the "auroral green line," is seen throughout the aurora down to about 100 kilometers, giving the aurora its dominant green appearance.

Atomic oxygen emission spectrum

Oxygen atoms emit 630 nanometer "auroral red line" light only at very high altitudes because these atoms are de-excited by collisions with nitrogen molecules below about 150 kilometers.

Atomic oxygen emission spectrum

The auroral green line disappears below 100 kilometers, where little atomic oxygen exists.

Graph of oxygen and nitrogen variation in the thermosphere

When highly energetic particles reach low levels, they trigger the red, blue, and violet emissions of molecular nitrogen, observed as a deep red or magenta border at the lower edge of auroral curtains. This dramatic effect is enhanced by the rapid motion associated with these very energetic displays. It is also possible to see waves of green light "chasing" the magenta, an effect caused by the time lag between excited oxygen and nitrogen. Oxygen atoms persist for about one second in their unstable state before decay, while the nitrogen molecule emissions are instantaneous.

Animation courtesy the COMET and HAO programs at UCAR/NCAR.

Last modified August 31, 2005 by Randy Russell.

Shop Windows to the Universe Science Store!

Learn about Earth and space science, and have fun while doing it! The games section of our online store includes a climate change card game and the Traveling Nitrogen game!

You might also be interested in:

Cool It! Game

Check out our online store - minerals, fossils, books, activities, jewelry, and household items!...more

Saturn's Aurora

Have you ever seen the Southern or Northern Lights? Did you know that other planets (besides Earth) have them too? Scientists call these cosmic light shows the "aurora". Saturn is one of the planets that...more

Charged Particle Motion in Earth's Magnetosphere

Motions within Earth's metallic core generate the planet's global magnetic field. This magnetic field extends beyond Earth's surface and atmosphere into the space surrounding our home planet. The interaction...more

Altocumulus

Altocumulus clouds are part of the Middle Cloud group. They are grayish-white with one part of the cloud darker than the other. Altocumulus clouds usually form in groups. Altocumulus clouds are about...more

Altostratus

Altostratus clouds belong to the Middle Cloud group. An altostratus cloud usually covers the whole sky. The cloud looks gray or blue-gray. The sun or moon may shine through an altostratus cloud, but will...more

Cirrocumulus

Cirrocumulus clouds belong to the High Cloud group. They are small rounded puffs that usually appear in long rows. Cirrocumulus are usually white, but sometimes appear gray. Cirrocumulus clouds are the...more

Cirrostratus

Cirrostratus clouds belong to the High Cloud group. They are sheetlike thin clouds that usually cover the entire sky. The sun or moon can shine through cirrostratus clouds. When looking at the sun through...more

Cirrus

Cirrus clouds are the most common of the High Cloud group. They are made of ice crystals and have long, thin, wispy streamers. Cirrus clouds are usually white and predict fair weather. ...more

Windows to the Universe, a project of the National Earth Science Teachers Association, is sponsored in part is sponsored in part through grants from federal agencies (NASA and NOAA), and partnerships with affiliated organizations, including the American Geophysical Union, the Howard Hughes Medical Institute, the Earth System Information Partnership, the American Meteorological Society, the National Center for Science Education, and TERC. The American Geophysical Union and the American Geosciences Institute are Windows to the Universe Founding Partners. NESTA welcomes new Institutional Affiliates in support of our ongoing programs, as well as collaborations on new projects. Contact NESTA for more information. NASA ESIP NCSE HHMI AGU AGI AMS NOAA