Auroras spotted far above Earth

SAN FRANCISCO — A military satellite has detected auroras, those shimmering displays of colorful light, at altitudes far higher above Earth than previously known, confirming anecdotal reports from astronauts that scientists had dismissed.

Known as the northern or southern lights, the auroras typically seen from the ground have been known to extend from about 60 miles to a few hundred miles above the planet’s surface.

Now, cameras on the Air Force’s Coriolis satellite launched early this year are also spotting auroras above 500 miles. How they are produced at such heights is still a mystery.

"It’s a relatively new aspect of this phenomenon no one has seen at this altitude," said David Webb, a research physicist at Boston College.

Details were presented Wednesday at the fall meeting of the American Geophysical Union.

Pulses of charged particles, mostly electrons, produce auroras when they are flung off by the sun and collide with molecules in the Earth’s atmosphere.

Previously, scientists believed the effect was limited to lower altitudes where the atmosphere is more dense. High above Earth, they believed, there simply weren’t enough air molecules to create the effect.

As a result, astronauts’ reports of seeing auroras weren’t believed, said Bernard V. Jackson, a solar physicist at the University of California, San Diego.

But something is there, scientists now say, and in sufficient quantities to create the newly spotted high-altitude auroras.

One theory is that the explosions of particles from the sun eject nitrogen from the ionosphere to much higher altitudes, Webb said.

Scientists first noticed the high-altitude auroras while looking at pictures from a trio of cameras included in the $10 million Solar Mass Ejection Imager. The experimental instrument was launched in January.

In images taken during a massive solar eruption in May, scientists observed a brightening that nearly swamped the instrument’s cameras.