Sun’s Magnetic Secret Revealed

By Jeremy Hsu
Staff Writer
posted: 22 January 2008
06:04 am ET

Powerful magnetic waves have been confirmed for the first time as major players in the process that makes the sun's atmosphere strangely hundreds of times hotter than its already superhot surface.

The magnetic waves — called Alfven waves — can carry enough energy from the sun's active surface to heat its atmosphere, or corona.

"The surface and corona are chock full of these things, and they're very energetic," said Bart de Pontieu, a physicist at the Lockheed Martin Solar and Astrophysics Laboratory in California.

The sun contains powerful heating and magnetic forces which drive the temperature to tens of thousands of degrees at the surface — yet the quieter corona wreathing the sun reaches temperatures of millions of degrees. Scientists have speculated that Alfven waves act as energy conveyor belts to heat the sun's atmosphere, but lacked the observational evidence to prove their theories.

De Pontieu and his colleagues changed that by using the Japanese orbiting solar observatory Hinode to peer at the region sandwiched between the sun's surface and corona, called the chromosphere. Not only did they spot many Alfven waves, but they also estimated the waves carried more than enough energy to sustain the corona's temperatures as well as to power the solar wind (charged particles that constantly stream out from the sun) to speeds of nearly 1 million mph.

However, the chromosphere findings alone could not prove the waves carried their energy into the sun's atmosphere.

"If you observe waves in the chromosphere, that doesn't mean they can get to the corona," De Pontieu told SPACE.com.

Some waves may get reflected back down to the sun instead of passing through the transition region between the surface and atmosphere. Waves that reach the corona also become more difficult to detect using current instruments, thanks to the long line-of-sight.

De Pontieu's group turned to researchers at the University of Oslo, Norway, who had created a computer simulation representing part of the sun. Once they knew what to look for, the researchers found magnetic waves within the simulation of the corona that strongly resembled the Alfven waves directly observed in the chromosphere.

Even as the simulations helped establish Alfven waves as energy carriers for the sun's atmosphere and solar wind, the new observational findings will help modelers create improved sun simulations.

"It goes back and forth — we learn from simulations, they learn from us," said De Pontieu.

Many mysteries remain about the sun's restless activities. De Pontieu's group focused on Alfven waves generated by the sun's heat turbulence, but other researchers examined Alfven waves generated when the sun's magnetic field lines stress and snap back together like invisible magnets. That reconnection force also creates jets of X-rays that shoot outwards from the sun, as captured by Hinode's instruments.

Scientists still don't know which source of Alfven waves plays a more important role in the heating the sun's atmosphere, but can use the latest findings as a stepping stone.

"We need to study both more, to see which one dominates," noted De Pontieu. "But it's nice for people to know that Alfven waves can do the job."