April's blog

WeatherQuesting

Q:  Why do oxygen atoms emit different colors at different altitudes during an aurora display?

An aurora taken from the International Space Station, as the ISS orbited earth at about the same altitude as the aurora.  From orbit, Pettit reported that flickering auroras appeared to crawl around like giant, green amoebas. Photo courtesy of Don Pettit, NASA.

During an aurora display, different atoms glow different colors (depending on their electrical state-ionized or neutral-and on the incoming particle's energy) to form vibrant, many-hued rings over Earth. Bombarded nitrogen ions shine blue and neutral nitrogen shines red. Oxygen atoms hit by incoming charged particles 200 miles high glow red, the rarest aurora color. At about 60 miles, glowing oxygen produces the most common color: a brilliant yellow-green.

An atom glows because the charged particle that hit it transferred kinetic energy to the atom. The atom often dumps the extra energy by emitting light and returns to its normal energy state. It glows like a neon atom inside a neon sign. The red and green oxygen emissions "...come from 'metastable' transitions, which means that once the oxygen atom enters an excited state, it sits there for a period of time before emitting a photon of light and returning to its original state," says Joe Hawkins, director of the Alaska Space Grant Program.

The excited red-energy state (6300 Angstroms) has a lifetime of about 110 seconds; whereas, the green (5577 Angstroms) has a lifetime of only 0.75 second. These widely discrepant times make a big difference because of collisions with neutral atoms. If a neutral atom bumps into an excited atom, the exited atom can change state before emitting its photon of light. So it doesn't emit light.

There's fewer atoms up high so they are less likely to collide. At 200 miles up, the exited red-energy state oxygen atoms have the time it takes for them to sit around for 110 seconds and then glow red. Down at 60 miles where oxygen atoms crowd together, however, a neutral atom is likely to hit a red-excited atom in the intervening 110 seconds so it can't glow red. A fast acting green-excited atom is better off. It only needs 0.75 second to emit its green photon and can likely do so without getting bumped by a neutral atom in the meantime. So it glows green.

That's the main reason. Another reason, says Hawkins, is a playoff between the availability of incoming electron "bullets" and of neutral atom "targets". At high altitude, there are few targets (because the atom density is low) and, at low altitude, few bullets (because of collision higher up). Thus, there is an intermediate altitude in which an optimum number of electron "bullets" and oxygen-atom "targets" exist to create the most excited atoms. That altitude for oxygen is 60 miles.

Further Reading

Aurora from space, NASA

(Answered July 25, 2001)