Have you ever seen the hot summer wind blow across a ripening field of wheat? If so, you’re familiar with the rippling effect. Now imagine that same crop – only the stalks are 32,000 feet high and on the surface of the Sun. This cascading effect is called Alfvén waves.
Thanks to NASA’s Solar Dynamics Observatory (SDO), we’re now able to see the effect of Alfvén waves, track their movements and see how much energy is being carried along. These new findings have enlightening solar researchers and may be the key to two other enigmatic solar occurrences – 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.
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“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.”
As McIntosh points out in his July 28 Nature article, Alfvén waves are pretty simple. Their movement undulates up and down the magnetic field lines similar to the way a vibration travels along a guitar string. The plasma field enveloping the Sun moves in harmony with the field lines. The SDO can “see” and track this movement. Although the scenario is much more complex, understanding the waves is key to understanding the nature of the Sun-Earth connection and other less clear cut questions such as what causes coronal heating and speeds of the solar wind.
“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.
Hannes Alfvén first theorized the waves in 1942, but it wasn’t until 2007 that they were actually observed. This proved they could carry energy from the Sun’s surface to the atmosphere, but the energy was too weak to account for the corona’s high heat. 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 “wiggles” – known as spicules – were then modeled against Alfvén waves and found to be a good match. Once pinpointed, the team could then 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.” says the team. “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.”
According to preliminary data, the spicules leaped to coronal temperatures of at least 1.8 million degrees Fahrenheit. The pairing of Alfvén waves and heat may just be what it takes to keep the corona at its current temperature… but not enough to cause radiation bursts. “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.”
More study? You betcha’. And the SDO team is up to the task.
“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.”
Original Story Source: NASA SDO News.