Magnetic waves bake the sun's corona

Why the sun's outer atmosphere is so blazing hot

5:30pm, July 27, 2011
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Scientists are getting warmer in their hunt for a reason why the sun’s outer atmosphere is so hot. The key may be magnetic waves long sought but only recently spotted, an international team reports in the July 28 Nature.

Combined with observations reported earlier this year (SN: 1/29/11, p. 12) of high-speed gas jets shooting up into the sun’s outer atmosphere, or corona, the magnetic waves may explain why the thin halo of superhot gas blazes at temperatures as high as a couple million kelvins. The waves may also account for the force behind the solar wind particles that stream off the corona at hundreds of kilometers per second.

“These are results that have been awaited for 50 years,” says Peter Cargill of Imperial College London and the University of St. Andrews in Scotland, who wasn’t involved with the work.

Solar physicists have struggled all that time to understand how the corona can be so hot when the layer below it, the chromosphere, is more than a hundred times cooler. One longstanding theory is that waves traveling through the sun’s magnetic field transport energy up from the seething solar surface. These oscillations move along magnetic field lines like vibrations on a plucked guitar string. In 2007 a team including Scott McIntosh of the National Center for Atmospheric Research in Boulder, Colo., finally detected these waves in the chromosphere, but the hint the researchers saw of waves in the corona couldn’t explain the heat surge.

Now, using a sensitive instrument aboard NASA’s Solar Dynamics Observatory, McIntosh and his colleagues have discovered magnetic waves throughout the corona and the transition zone between it and the chromosphere.

“What surprises me is that they are seen absolutely everywhere and with energy levels that are able to do things to the corona,” Cargill says.

The waves wiggle at about the same rate as the ones in the chromosphere (they may even be the same waves, propagating upward). The energy supplied by the waves — the equivalent of one or two 100-watt bulbs per square meter of the sun’s surface — isn’t much, but is enough to power the solar wind and explain the energy pouring out of tranquil parts of the corona where things like flares aren’t happening. The waves can’t account for energy in active spots, but that might just be a byproduct of the way the spacecraft looks at the corona, Cargill and Ineke De Moortel of the University of St. Andrews suggest in a commentary published in the same Nature issue.

McIntosh’s team thinks there may be a two-step heating process happening. Solar magnetic activity may heat the jets, which then shoot upward, carrying their heat and the waves with them like the space shuttle’s solid rocket boosters shoot the whole thing into the sky, he says. In the corona the magnetic waves kick in, “like the main engine for the final push,” and deposit their energy in the gas. Exactly how the waves transfer energy to the corona isn’t clear.

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