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Mystery of Sun's Superhot Corona May Be Whipped

Rippling magnetic fields finally spotted in the corona
corona



GNU PUBLIC LICENSE: LUC VIATOUR
New measurements provide the strongest evidence yet for wriggling magnetic waves that may explain why the solar corona is a good 200 times hotter than the surface of the sun: regular pulsations in the speed of charged, high-energy gas or plasma streaming from the sun combined with matching magnetic fields.

The findings point to magnetic field ripples known as Alfvén waves whipping plasma back and forth, releasing heat, according to a report published online by Science. Although the waves seem to be too tiny to transfer the energy needed to fully heat the corona, researchers say improved measurements may reveal that the waves are larger than they appear.

A central problem in solar physics is why the sun's atmosphere gets hotter as it rises, from about 8,500 degrees Fahrenheit (5,000 kelvins) at the photosphere (solar surface) to more than 1.8 million degrees Fahrenheit (1 million Kelvins) in the corona, which extends for millions of miles. Without an outside force heating the corona, its temperature would plummet, says solar physicist Scott McIntosh of the National Center for Atmospheric Research (NCAR) in Boulder, Colo.

A top candidate for that outside force: Alfv&eacutise;n waves, undulations in the corona's magnetic field liberating heat as the waves "break" and compress plasma.

To spy Alfvén waves, McIntosh, Steven Tomczyk, also of NCAR, and their colleagues used the coronal multichannel polarimeter (CoMP) instrument at New Mexico's National Solar Observatory to measure changes in light emitted by ionized iron in the corona, which told them how the plasma's outward velocity varied.

Pulses of 0.2 mile (0.3 kilometer) a second occurred every five minutes, the same frequency as convective currents in the photosphere that could trigger Alfvén waves, they report. The direction of coronal magnetic field lines also seemed to match the trajectories of the pulses.

The result comes close to solving one of the great mysteries in solar physics, says Viggo Hansteen, professor of astrophysics at the University of Oslo in Norway. Data from Japan's Hinode spacecraft, presented at a meeting in April, showed patterns deeper in the corona that seemed to finger Alfvén waves, but the new finding is much more definitive, he says. "They're not guessing. They're really saying, 'Okay, these things are obeying all the characteristics we expect of Alfvén waves.'"

The only hitch: the waves are less than a thousandth the mangnitude needed to explain coronal heating, but Tomczyk notes that CoMP has relatively low resolution, so much larger waves could still be hidden in the data.

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