It's hard to explain how a surface so relatively chill can energize an atmosphere that is so hot.
"It's like placing something in the fridge and it boils," said Leon Ofman, a solar physicist at NASA's Goddard Space Flight Center in Greenbelt Md. "It doesn't make sense."
Theorists have blistered their brains, and solar astronomers have built sophisticated telescopes and spacecraft in pursuit of this and other solar mysteries.
Now a solar-orbiting telescope designed to study the Sun's corona has captured images that are allowing scientists to answer the riddle of the corona's heat.
A paper published in this week's issue of the journal Science suggests an answer to the conundrum. The explanation may be that the corona is much stiffer than longstanding theory predicts.
The corona might actually behave more like thick dough than high-performance engine oil, said Ofman, who is one of the five authors of the new article.
The finding is simultaneously a big surprise and a perfectly understandable result. It raises eyebrows because physicists believe that plasma at coronal temperatures should be extremely fluid and flow easily. At the same time, the possibility that the corona is stiff fits well with what scientists have observed about its behavior and the way it traps heat.
The new interpretation is based on analysis of solar images taken last summer through the TRACE spacecraft's 12-inch (30-centimeter) telescope. TRACE returns hundreds of images to Earth each day.
When solar flares violently erupt into the corona, they radiate shock through the plasma. The pictures studied by Ofman and his colleagues show the shock wave from a solar flare hitting towering arches of hot gas called coronal loops. (Click on the link at the top of this story to view the moving image.) The shock of the eruption causes the loops to vibrate, a reaction Ofman described as similar to the vibration of a plucked guitar string.
Richard Fisher, also a solar physicist at Goddard, and mission scientist for TRACE, describes the event in more explosive terms. "It's like you set off a cherry bomb a meter away from a slinky and the slinky wiggles when it's whacked," Fisher said.
Only in this case, the cherry bomb is more than 60,000 miles (100,000 kilometers) away from a 105,000-mile-long (170,000 kilometer-long) slinky. That wound up coronal loop would stretch about 35 times the distance from San Diego, Calif. to Portland, Maine.
When the solar flare shocked the coronal loop, the immense arch started swinging back and forth at a rate of once every four minutes. Within about an hour, though, the vibration had completely subsided, a fact that surprised Ofman and his co-authors.
"If the viscosity (of the coronal plasma) was as low as initially thought, the oscillation should go on for a much longer time, maybe a thousand hours" Ofman said.
Viscosity is a measure of a fluid's resistance to force. It is harder to shake a stick in mud than in water because mud has a higher viscosity.
The quick damping of the coronal loop's vibration means that the corona must be much thicker than previously thought, he said. If this is true, it may explain the corona's tremendous heat.
A thick corona would trap heat and provide a mechanism for frictional heat to buildup, but this possibility forces scientists to address the question of how the corona could be so stiff.
What could explain the high viscosity, Ofman thinks, is turbulence. The coronal plasma may be extremely dynamic, with tremendous small-scale turbulence throughout. Just as it is easier to swim through still water than it is through a pool being churned by a hundred jacuzzi jets, a turbulent corona would have a higher viscosity.
The significance of the finding, Ofman said, is that it helps physicists better understand the corona, which is crucial to learning more about the Sun-Earth connection.
Solar weather and events such as solar flares strongly affect Earth, but scientists cannot predict with any reliability how these phenomena will affect Earth. A more complete understanding of the corona will someday help forecasters to better predict how events on the Sun affect Earth.
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