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Wave power key
to Sun's hot corona

by Dr Emma Rigby
Posted: 01 August 2011

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New results from NASA's Solar Dynamics Observatory (SDO), published in the journal Nature, may have solved the mystery of how the Sun's outer atmosphere can be more than twenty times hotter than its surface, and could lead to a better understanding of the intense solar wind and its impact on Earth.

Spicules, giant plasma fountains shooting up from the Sun, as seen by the SDO on 25 April 2010; they sway like underwater seaweed thanks to ripples in the solar magnetic field. Image: NASA/SDO/AIA.

The surface regions of the Sun are linked to its outer atmosphere, or corona, by spicules – towering fountains of plasma which can stretch upwards for 32,000 miles. Magnetic field ripples called Alfvén waves cause the spicules to sway back and forth, in a similar way to seaweed in moving water. These waves were long suspected of being the mechanism responsible for carrying energy up into the corona, but when they were directly observed for the first time four years ago they were too weak. However, new images and movies from the recently launched SDO have revealed waves over a hundred times stronger than those seen in 2007.

"SDO has amazing resolution so you can actually see individual waves," says the study's lead scientist Scott McIntosh of the National Center for Atmospheric Research in Colorado. "Now we can see that instead of these waves having about 1000th the energy needed as we previously thought, it has the equivalent of about 1100W light bulb for every 11 square feet of the Sun's surface, which is enough to heat the Sun's atmosphere and drive the solar wind. It’s like getting a microscope to study the Sun’s corona, giving us the spatial and temperature coverage to focus in on the way mass and energy circulate.”

The energy transported by the recently seen Alfvén waves may also be enough to drive the solar wind. This stream of charged particles, blasted out into the Solar System at up to 1.5 million miles per hour, can cause geomagnetic storms on Earth and disrupt technology such as telecommunications satellites.

Now that these observations have revealed the true power of the Alfvén waves, the team can develop detailed computer simulations to investigate exactly how the energy they bring is used in the corona. "Knowing there may be enough energy in the waves is only one half of the problem," says Vladimir Airapetian from NASA's Goddard Space Flight Centre. "The next question is to find out what fraction of that energy is converted into heat. It could be all of it, or it could be 20 percent of it – so we need to know the details of that conversion."

"We still don't perfectly understand the process going on, but we're getting better and better observations," adds McIntosh. "The next step is for people to improve the theories and models to really capture the essence of the physics that's happening."