Like giant strands of seaweed some 32,000 miles high, material shooting up from the sun sways back and forth with the atmosphere. In the ocean, it's moving water that pulls the seaweed along for a ride; in the sun's corona, magnetic field ripples called Alfvén waves cause the swaying.
For years these waves were too difficult to detect directly, but NASA's Solar Dynamics Observatory (SDO) is now able to track the movements of this solar "seaweed" and measure how much energy is carried by the Alfvén waves. The research shows that the waves carry more energy than previously thought, and possibly enough to drive two solar phenomena whose causes remain points of debate: the intense heating of the corona to some 20 times hotter than the sun's surface and solar winds that blast up to 1.5 million miles per hour.
"SDO has amazing resolution so you can actually see individual waves," says Scott McIntosh at the National Center for Atmospheric Research in Boulder, Colo. "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."
McIntosh published his research in a Nature article appearing on July 28. Alfvén waves, he says, are actually fairly simple. They are waves that travel up and down a magnetic field line much the way a wave travels up and down a plucked string. The material surrounding the sun -- electrified gas called plasma -- moves in concert with magnetic fields. SDO can see this material in motion and so can track the Alfvén waves.
Alfvén waves are part of a much more complex system of magnetic fields and plasma surrounding the sun. Understanding that system could help answer general questions such as what initiates geomagnetic storms near Earth and more focused questions such as what causes coronal heating and speeds of the solar wind -- a field of inquiry in which there are few agreed-upon answers.
"We know there are mechanisms that supply a huge reservoir of energy at the sun's surface," says space scientist Vladimir Airapetian at NASA's Goddard Space Flight Center in Greenbelt, Md. "This energy is pumped into magnetic field energy, carried up into the sun's atmosphere and then released as heat." But determining the details of this mechanism has long been debated. Airapetian points out that a study like this confirms Alfvén waves may be part of that process, but that even with SDO we do not yet have the imaging resolution to prove it definitively.
When the waves were first observed in 2007 (more than six decades after being hypothesized by Hannes Alfvén in 1942), it was clear that they could in theory carry energy up from the sun's surface to its atmosphere. However, the 2007 observations showed them to be too weak to contain the great amounts of energy needed to heat the corona so dramatically.
This study says that those original numbers may have been underestimated. McIntosh, in collaboration with a team from Lockheed Martin, Norway's University of Oslo, and Belgium's Catholic University of Leuven, analyzed the great oscillations in movies from SDO's Atmospheric Imagine Assembly (AIA) instrument captured on April 25, 2010.
"Our code name for this research was 'The Wiggles,'" says McIntosh. "Because the movies really look like the sun was made of Jell-O wiggling back and forth everywhere. Clearly, these wiggles carry energy."
The team tracked the motions of this wiggly material spewing up -- in great jets known as spicules -- as well as how much the spicules sway back and forth. They compared these observations to models of how such material would behave if undergoing motion from the Alfvén waves and found them to be a good match.
Going forward, they could analyze the shape, speed, and energy of the waves. The sinusoidal curves deviated outward at speeds of over 30 miles per second and repeated themselves every 150 to 550 seconds. These speeds mean the waves would be energetic enough to accelerate the fast solar wind and heat the quiet corona. The shortness of the repetition -- known as the period of the wave -- is also important. The shorter the period, the easier it is for the wave to release its energy into the coronal atmosphere, a crucial step in the process.
Earlier work with this same data also showed that the spicules achieved coronal temperatures of at least 1.8 million degrees Fahrenheit. Together the heat and Alfvén waves do seem to have enough energy to keep the roiling corona so hot. The energy is not quite enough to account for the largest bursts of radiation in the corona, however.
"Knowing there may be enough energy in the waves is only one half of the problem," says Goddard's Airapetian. "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."
In practice, that means studying more about the waves to understand just how they impart their energy into the surrounding atmosphere.
"We still don't perfectly understand the process going on, but we're getting better and better observations," says McIntosh. "The next step is for people to improve the theories and models to really capture the essence of the physics that's happening."
- Scott W. McIntosh, Bart De Pontieu, Mats Carlsson, Viggo Hansteen, Paul Boerner, Marcel Goossens. Alfvénic waves with sufficient energy to power the quiet solar corona and fast solar wind. Nature, 2011; 475 (7357): 477 DOI: 10.1038/nature10235
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