Illustration courtesy Wedemeyer-Böhm and VAPOR
A telescope picture of a solar tornado. Image courtesy Scullion, Wedemeyer-Böhm et al.
Published June 27, 2012
If Dorothy lived on the sun, she'd need truckloads of ruby slippers.
According to a new study, 11,000 magnetic supertornadoes may cover our star at any moment—each possibly as large as the United States.
Watch a simulation of a solar supertornado.
A team of solar researchers discovered 14 of the supersize tornadoes using a telescope in space and one on the ground.
Made of swirling, searing-hot plasma, each twister stretched roughly 1,800 miles (2,900 kilometers) high and spun about 9,000 miles (14,500 kilometers) an hour. (Related: "Giant Solar Tornado Caught in NASA Video.")
The team saw the first signs of solar tornadoes in 2008 but couldn't confirm their existence until now.
"We observed some unusually hot plasma above the sun's surface, so we knew something was happening there, but we didn't know what," said study co-author Sven Wedemeyer-Böhm, an astrophysicist at the University of Oslo.
Extrapolating from the portion of solar atmosphere they studied, the team calculated that around 11,000 of the twisters exist across the sun at any time.
Based on their new computer simulations of the tempests, the researchers also think solar tornadoes may be the keys to a long-standing mystery: Why is the sun's upper atmosphere 300 times hotter than its surface?
A Hot Problem
The sun's faint upper atmosphere, or corona, is visible from Earth during a total solar eclipse, when the moon blocks out the disk of the sun. (Related: "Eclipse Expert Makes Hot Finds in Sun's Darkest Hour.")
In 1939 solar researchers used an eclipse to sample the corona's light and peg its temperature at 3.6 million degrees Fahrenheit (2 million degrees Celsius)—far hotter than the surface temperature of 10,000 degrees Fahrenheit (5,500 degrees Celsius).
"You'd expect the sun's temperature to decrease outwards, but that's just not the case," Wedemeyer-Böhm said.
"Something has to transport heat energy there," he added. But what?
Other than solar twisters, two ideas stand out. One posits that countless "nano" solar flares—theoretically invisible with current instruments—are constantly launching spaceward and heating the corona.
Another theory is that ripples of energy called Alfvén waves travel at 9 million miles (14.5 million kilometers) an hour along the sun's magnetic field lines, shuttling energy into the corona. (See "'Corkscrew' Waves Seen on Sun—Keys to Solar Mystery?")
Solar tornadoes provide a grandiose new way to get the job done and may incorporate a bit of both ideas.
The team developed computer models to explain the structures of solar tornadoes, and their results closely match actual observations of the sun taken by the Swedish 1-m Solar Telescope and NASA's space-based Solar Dynamics Observatory.
Based on their models, Wedemeyer-Böhm and colleagues think solar tornadoes form when hot surface particles surge up into the atmosphere and then sink back down.
The downward motion of the particles rotates the sun's magnetic field lines, forming vortices.
Each spinning mess of field lines can extend thousands of kilometers into the star's corona, carrying hot surface plasma along for the ride. These solar supertornadoes can last about 13 minutes each.
Tornadoes on Other Stars?
For now, though, the team lacks enough data to say too much about solar tornadoes, Wedemeyer-Böhm said. In particular, it's not known if solar tornado numbers and intensity shift during the sun's 11-year-long space-weather cycle.
It could be that the team saw a glut of huge tornadoes in recent observations because the sun has been ramping up toward maximum activity levels in the current cycle.
Solar physicist Sergei Zharkov of University College London agreed, saying that he'd like to see much more data on solar tornadoes to estimate their heating effect on the corona.
"[It's] likely not enough to generalize for the entire sun going through different stages of its activity cycle," said Zharkov, who wasn't involved in the study. "But of course, this is still an excellent first estimate."
Wedemeyer-Böhm's team is now working on gathering more observations of the sun's tornadoes. Until then, the results of their computer simulations are being used to look beyond our solar system.
"There are implications for stars other than the sun," Wedemeyer-Böhm said. "We have modeled the surfaces of stars about half as cool as the sun, and the simulations for those also produced magnetic tornadoes."
The new solar supertornadoes are described in this week's issue of the journal Nature.
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