Normally the solar atmosphere is calm, but its wind can carry "space weather" from solar flares or other disturbances on the sun's surface.
These cause auroras, disrupt satellites and radio communications, and—in extreme cases—wreak havoc with power grids.
(Read "The Sun—Living With a Stormy Star" in National Geographic Magazine [July 2004].)
Understanding solar wind could help scientists better predict the impacts of space weather—but the exact source of the winds has long been a mystery.
In 1947 the physicist Alfvén published a paper in the journal Nature proposing the existence of electromagnetic-hydromagnetic waves.
Since then the waves have been produced in the lab, but it was not until recently that researchers could detect the waves in the sun's corona.
The new studies show that Alfvén waves occur when turbulence at or near the sun's surface causes magnetic field lines to produce wiggles that propagate outward.
"They're kind of like waves on a guitar string, in the sense that if you pluck a guitar string, the wave travels along [it]," de Pontieu said.
His team was able to spot the waves by looking at the motions of "spicules" in the chromosphere, a thin layer of the sun's atmosphere.
Spicules are jets of hot gas that shoot outward from the sun's surface at speeds of 100,000 miles (160,000 kilometers) an hour, reaching heights of 5,000 miles (8,000 kilometers) in a matter of minutes.
Using Hinode's high-speed camera, the scientists were able to photograph spicules every five seconds.
The jets are lined up along the sun's magnetic field lines, which stick out from the star's surface.
Alfvén waves make the spicules that form along the field lines seem to "dance," somewhat like a person wiggling a magnet in the middle of an array of iron filings.
Similarly, the team led by NASA's Cirtain looked at the sun's x-ray emissions, particularly from high-energy jets near its magnetic poles.
These showed that Alfvén waves are created when tangled magnetic field lines "short circuit" and snap into new configurations. That causes the field lines to vibrate in response to the large quantities of energy being released.
Alfvén waves on the sun last for about ten minutes each, and Hinode's x-ray telescope allowed a new image to be taken every 30 seconds.
"We could directly observe the waves" moving in the images, Cirtain said.
Still another team, led by Takenori Okamoto of Japan's National Astronomical Observatory, was able to see similar effects in prominences, which are large plumes of gas rising hundreds of thousands of miles above the sun's surface.
Unlike spicules and x-ray jets, which lie near the surface, prominences extend into the sun's corona, where they can persist for many days.
As the Alfvén waves rise above the sun's surface, they impart energy to gases in the sun's corona, propelling them outward.
"[Alfvén waves] are very good at propagating through the [sun's] atmosphere because they don't get damped easily," Lockheed Martin's de Pontieu said. "They carry a lot of energy."
Not all of this energy reaches the corona, but mathematical simulations by de Pontieu's team indicate that enough does to accelerate the solar wind.
"There's kind of a wall between the chromosphere and the corona, and a lot of the waves get reflected," de Pontieu said.
"But we've been able to show that enough get into the corona to provide the energy to drive the solar wind."
The new studies don't allow for prediction of solar flares and other disruptive explosions from the sun, the researchers caution.
"We're helping theorists develop better models of the formation of the solar wind," NASA's Cirtain said.
"The increased precision and accuracy of those models will allow solar weather forecasters to create a more accurate representation of the output of the sun on a day-to-day basis."
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