Untangling the solar corona's structure.

The cauldron of gases at the sun's visible surface measures 6,000 kelvins. But venture a few thousand kilometers higher, and temperatures typically zoom into the millions of kelvins. Welcome to the solar corona, where vast, arching magnetic fields hold sway and bursts of radiation can occasionally exceed the temperature at the sun's very core.

Scientists have tried for more than half a century to explain how the corona retains its heat. But they lacked the observational tools to discriminate clearly among various models. Examining new X-ray images obtained by the Japanese Yohkoh satellite, however, two researchers argue that coronal heat comes from energy released when tangles of magnetic fields unwind.

James A. Klimchuk of the Naval Research Laboratory in Washington, D.C., and Lisa J. Porter of the Massachusetts Institute of Technology report in the Sept. 14 Nature their analysis of Yohkoh images of 47 loops of coronal gas sculpted by the arching magnetic fields.

Klimchuk and Porter note that in recent years, three models have taken center stage in the effort to explain the corona's high temperatures. In one model, wavelike motions that originate at the sun's turbulent surface propagate along magnetic field lines into the corona, where they act as a steady source of energy.

The other two models rely on energy generated when the corona's magnetic fields--either singly or in groups--twist or tangle. The twisting arises because these arching fields begin and end at "footpoints" in the sun's visible surface. The turbulence at the surface causes the footpoints to wander, tangling the magnetic fields.

In one of these models, individual fields become so twisted that they eventually snap like rubber bands, unleashing energy fields. In the other, neighboring fields wind around each other, creating what the researchers call "a bowl of tangled spaghetti." The corona's heat comes from energy released as the spaghetti tries to untangle itself, while the footpoint motion continues to stir it up.

In their study, Klimchuk and Porter deduced from Yohkoh images that a coronal gas loop gains heat at a rate inversely proportional to the square of the loop's length. Combining this observation with earlier results from solar physicist Leon Golub of the Harvard-Smithsonian Astrophysical Observatory in Cambridge, Mass., and his colleagues, who analyzed 1970s Skylab data to relate pressure in the corona to magnetic field strength, Klimchuk and Porter say their work supports the heating predicted by the spaghetti model. But because no one else has done a study similar to Golub's, the two researchers caution that "it would be premature to rule out other models on the basis of this evidence alone."

"We clearly need more data, but [this] work is showing us a way to go about learning how the corona is heated," says Robert Rosner of the University of Chicago.

According to Golub, X-ray telescopes aboard SOHO, a joint NASA--European Space Agency mission scheduled for launch in December, may provide further data on coronal heating. And a NASA small-explorer project called TRACE, set for launch in 1997, will have detectors with five times the resolution of any existing low-energy X-ray telescope, notes Rosner.

Researchers may already have found direct evidence of footpoints twisting the magnetic field, says Theodore D. Tarbell of Lockheed Palo Alto (Calif.) Research Laboratory. He and his colleagues used the Swedish Vacuum Solar Telescope in the Canary Islands, Spain, to show that tiny magnetic elements on the sun's surface, apparently linked to coronal gas loops, change shape in a matter of minutes.