In the eye of the beholder. Sharper views of the sun at a variety of wavelengths are revealing small jets from the solar surface that are helping heat the overlying corona to 1 million˚C.

Bart De Pontieu

Really Hot Doin's Discovered on the Sun

The mystery of the solar corona is obvious enough. The vanishingly thin atmosphere of the sun—the wispy stuff that can be glimpsed faintly during total solar eclipses—simmers at 1 million˚C, 200 times hotter than the "fire" beneath it. What gives? Researchers now believe they have caught the sun in the act of heating bits of itself to coronal temperatures and jetting those bits up into the corona.

Researchers have had plenty of ideas about why the corona is so hot but no convincing way to test them. Whatever the process, it was happening on too fine a spatial scale for instruments to discern clearly. But that changed with the data dumps following the launches of the Japanese-led Hinode mission in 2006 and NASA's Solar Dynamics Observatory (SDO) in February of last year. At visible wavelengths, Hinode's imager can resolve features on the solar surface as small as 150 kilometers. At the same time, an extreme ultraviolet imager on SDO can follow small features at eight different ultraviolet wavelengths that gauge temperatures from 20,000˚C to 1 million˚C. SDO images the whole face of the sun every 12 seconds, sending back 1.5 terabytes of data each day.

The combination of Hinode and SDO observations has now shown at least one way the corona gets heated. Solar physicist Bart De Pontieu of the Lockheed Martin Solar and Astrophysics Laboratory in Palo Alto, California, and his colleagues report in the 7 January issue of Science that they can trace jets of plasma, or ionized gas, rising into the corona. The jets ascend at 180,000 to 360,000 kilometers per hour from 300-kilometer-wide bright spots on the surface called spicules. Within its less-than-2-minute lifetime, a jet's temperature soars, some parts reaching corona-like temperatures of about a million degrees. And, most telling, "we found them everywhere," says De Pontieu, and "they go up, but they don't come down."

A rough estimate suggests that the energy that these roaring jets deliver to the corona might well account for its heating, says De Pontieu. "We're not saying it's the dominant mode," he adds. "We are saying it's time to reconsider what kinds of processes are involved." Many theorists had placed their heating mechanisms up in the corona itself, but De Pontieu sees the focus moving down to near the sun's visible surface where these jets originate.

"They've discovered a new phenomenon," says solar astrophysicist Spiro Antiochos of NASA's Goddard Space Flight Center in Greenbelt, Maryland. The pervasiveness of the hot, corona-penetrating jets "doesn't prove their phenomenon is the only one heating the corona. It does show it's a ubiquitous phenomenon. That's one test." The next test should be more detailed modeling, Antiochos says, followed by even better observations using NASA's Interface Region Imaging Spectrograph, to be launched in December 2012. Modeling and new observations might even reveal what drives these jets.