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Hotter than the sun

Researchers directly observe Alfvén waves, which keep the corona sizzling

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1:07pm, March 19, 2009
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Magnetic waves theorized to transfer heat from the surface of the sun to its atmosphere have been directly observed for the first time, researchers report in the March 20 Science.

Astrophysicists have long puzzled over why the sun’s corona, the outer part of the solar atmosphere, is millions of degrees hotter than the surface of the sun itself. “It’s counterintuitive — when you hold your hands in front of a fire, it’s hottest closest to the flames,” says David Jess, an astrophysicist at Queen’s University Belfast, in Northern Ireland, and a study coauthor.

The magnetic waves, called Alfvén waves, are considered the most plausible explanation for the transfer of so much energy from the sun’s surface to its outer atmosphere. First theorized by Nobel laureate Hannes Alfvén in 1942, the waves could carry energy several hundred thousand kilometers from the sun’s surface to the corona.

The new observation “is quite interesting,” says astrophysicist Craig DeForest of the Southwest Research Institute in Boulder, Colo. “It means that we can get to the root of what’s heating the corona.”

Alfvén waves move along the sun’s magnetic fields like “waves traveling along a string,” Jess explains. The waves are created by magnetic reconnections, disturbances in the sun’s magnetic field created when magnetic lines twist, break apart and then snap back together again.

Scientists use powerful telescopes to study the surface of the sun, but proving detected oscillations are Alfvén waves has been a difficult task.

“Looking at the sun through a telescope is like a fish at the bottom of a stream trying to examine feathers on a bird overhead,” DeForest says. “Our atmosphere distorts the image much as the surface of a stream would.” 

Until now, researchers were able to use only theoretical models built on small amounts of data to predict how the waves would act. But in the new study, Jess and his colleagues used the Swedish 1-m Solar Telescope to observe a single, highly magnetized bright point on the surface of the sun and measure the Alfvén waves oscillating from that point. A combination of technological advances and good atmospheric conditions also allowed the researchers to gather a large amount of data on the waves.

The data suggest that the waves are energetic enough to heat the whole corona. “The next logical step is to measure how much energy the waves produce at different places in the sun’s atmosphere to figure out if Alfvén waves are the dominant mechanism for heating the whole solar atmosphere,” Jess says.

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