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The Solar Dynamics Observatory spots extra energy in the Sun's coronaScientists say these magnetic field ripples could be the source of the corona’s intense heating and solar winds.
By
NASA's Goddard Space Flight Center, Greenbelt, Maryland —
Published:
July 28, 2011 These jets, known as spicules, were captured in an SDO image April 25, 2010. Combined with the energy from ripples in the magnetic field, they may contain enough energy to power the solar wind that streams from the Sun toward Earth at 1.5 million miles (2.4 million kilometers) per hour. Image credit: NASA/SDO/AIA
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 about 20 times hotter than the Sun's surface, and solar winds that blast up to 1.5 million mph (2.4 million km/h).
"SDO has amazing resolution, so you can actually see individual waves," said Scott McIntosh from the National Center for Atmospheric Research in Boulder, Colorado. "Now, we can see that instead of these waves having about one-thousandth the energy needed as we previously thought, it has the equivalent of about an 1,100 watt light bulb for every 11 square feet (1 square meter) of the Sun's surface, which is enough to heat the Sun's atmosphere and drive the solar wind."
Alfvén waves, McIntosh said, 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 therefore can track the Alfvén waves.
Alfvén waves are part of a 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, as well as 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," said Vladimir Airapetian from NASA's Goddard Space Flight Center in Greenbelt, Maryland. "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 scientists do not yet have the imaging resolution to prove it definitively.
When the waves were first observed in 2007 — more than 6 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 April 25, 2010.
"Our code name for this research was 'The Wiggles,'" said 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 (48 km) 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 (1.0 million degrees Celsius). 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," said 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," said McIntosh. "The next step is for people to improve the theories and models to really capture the essence of the physics that's happening."
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