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New kink in sun’s strange corona

Image: Sun's hot corona
Erupting clouds of plasma are suspended in the sun's hot corona. Every feature in this image traces magnetic field structure.ESA
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Here's a strange scenario: You move farther away from a fire, getting cooler and cooler, until suddenly you are burning up.

That's essentially what happens in the sun: Its outer layer, the corona, is inexplicably hot. A new study may complicate things further by poking holes in a leading theory that aims to account for the puzzling phenomenon.

Last year, astrophysicist Steve Tomczyk of the National Center for Atmospheric Research in Boulder, Colo., and his colleagues asserted that corkscrew-shaped Alfven waves were converting the motion energy of the sun's roiling material into heat.

But the authors of the new study argue that the waves Tomczyk's team saw were not Alfven waves but kink waves.

"Kink waves look like kinks in hair or rope," said University of Warwick astrophysicist Tom Van Doorsselaere, one of the researchers behind the new study. "Kink waves can't explain why the corona is so hot. They carry less energy with them."

Van Doorsselaere said he and his colleagues used a more complex model than Tomczyk and found that the wave observations are not consistent with Alfven waves, and must be kink waves.

"At the moment I can't see any other explanation that could explain the observations," Van Doorsselaere told "I would have wished the other authors were right because it would have been good news to finally solve the problem."

The new study was published March 6, 2008, in the Astrophysical Journal Letters.

Van Doorsselaere said Tomczyk and his colleagues still think Alfven waves could be behind the corona's heat.

The corkscrewing motion of the Alfven oscillation, left, and the kinking motion of the kink oscillation, right.

"They're not convinced that we are right, but they are convinced that our opinion should be published and discussed," he said. "Probably at upcoming conferences, there will be some lively discussion of this."

The sun is not alone in having an inexplicably scorching outer layer.

"We think our sun is pretty typical for corona behavior," Van Doorsselaere said. "There are other stars with even bigger and more active coronas."

Our sun, being so close to us, provides the best opportunity to study this phenomenon. Scientists still hold out hope we can get to the bottom of the mystery soon.

"To test this we need better observations," Van Doorsselaere said. "We're getting better telescopes and satellites to look at the sun. There are several missions coming up to hopefully resolve the problem."

He said NASA's upcoming Solar Dynamics Observatory, to be launched sometime at the end of 2008 or beginning of 2009, or ESA's Solar Orbiter, set to launch in 2015, could be the keys to success.