Sun's "Ring of Fire" Stoked by Sound Waves

Stefan Lovgren
for National Geographic News

May 30, 2007

It has been a burning mystery for decades: Why is a thin, irregular region of the sun's atmosphere known as the chromosphere much hotter than the star's visible surface?

The answer, scientists recently proposed, could be stellar sound.

The temperature at the sun's surface layer, or photosphere, is about 10,000°F (6,000°C)—much cooler than its 27,000,000°F (15,000,000°C) interior.

But in the chromosphere, the region just above the photosphere, the heat spikes again to about 20,000°F (11,000°C).

"It's like getting warmer as you move away from a fire instead of cooler—certainly not what you expect," said Scott McIntosh, a researcher at Southwest Research Institute in Boulder, Colorado.

Now McIntosh and colleagues' studies show that magnetic fields send sound waves from the sun's interior shooting upward, creating fountains of hot gas that shape and power the chromosphere.

The researchers compared the phenomenon to standing in Yellowstone National Park and being surrounded by musical geysers that pop up at random, sending shrill notes and hot water shooting high into the air.

(Related: watch movies of solar structures in action, including images of sound waves moving through the sun.)

"There's a lot more wave energy leaking into the solar chromosphere than we previously thought," McIntosh said on Tuesday during a meeting of the American Astronomical Society (AAS) in Honolulu, Hawaii.

Sound and Fury

The chromosphere, or "color sphere," is so named because its higher temperatures cause the sun's hydrogen to emit reddish light.

Clouds of material from the chromosphere suspended above the sun can create a ruby-red "ring of fire" around a total solar eclipse.

The chromosphere is largely responsible for the deep ultraviolet radiation that hits Earth, producing our atmosphere's ozone layer.

The region might also contribute to climate change on Earth, researchers say (related: "Don't Blame Sun for Global Warming, Study Says" [September 13, 2006]).

Previously scientists have proposed either sound waves or the ever changing solar magnetic fields as potential causes of the chromosphere's unexpected heat.

The latest study shows "that the magnetic field and the sound wave go hand in hand to create a lot of the … mass and energy of the chromosphere," Bart De Pontieu, a researcher at the Lockheed Martin Solar and Astrophysics Lab in Palo Alto, California, said at the meeting.

The sun's interior vibrates with millions of sound waves, which are inaudible to humans but pack much more power than regular sound waves.

These waves are largely trapped by the photosphere.

Using spacecraft, ground-based telescopes, and computer simulations, scientists found that some of the trapped waves and their pent-up energy manage to leak out through magnetic "knots" in the photosphere.

The leaks send mass and energy streaming upward in thin fountains into the chromosphere.

"What we've discovered is exactly how these waves can leak out from the sun's interior through … naturally occurring magnetic portals," Southwest Research Institute's McIntosh said.

Scientists at the meeting said that the findings will help solar astronomers study other stars.

"We astronomers look at the sun in a very special way—it's the only star we can look at it in very high detail," said Andrea Dupree, a former AAS president.

"The lessons we're learning from the sun," Dupree said, "are ones we're going to take with us and look at all the other stars and think about how these other stars work."

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