SOHO craft helps solve a solar mystery.

For more than 50 years, a simple property of the sun's outer atmosphere, the corona, has mystified astronomers. Though it lies thousands of kilometers above the surface of the sun, the corona is much hotter, with an average temperature hundreds of times as great.

It would require only a tiny fraction of the sun's energy to fire up the corona to several million kelvins, but ordinary heat can't flow from a cooler region to a hotter one (SN: 8/31/96, p. 136). Scientists have suggested instead that a sudden release of energy stored in the sun's magnetic field could do the trick, but they lacked compelling evidence. New findings from a spacecraft that stares continuously at the sun may go a long way to providing that proof.

Observations with the Solar and Heliospheric Observatory (SOHO) reveal that the visible surface of the sun is carpeted with tens of thousands of magnetic field bundles that loop upward into the corona. Each bundle is composed of pairs of oppositely directed field lines that about every 40 hours, merge and annihilate each other releasing vast amounts of energy. New bundles then emerge to replenish the magnetic carpet.

SOHO has also found that in a region of the solar atmosphere just below the corona--some 1,600 km above the sun's surface-the brightest features lie directly above the highest concentration of magnetic bundles at the surface.

"This is direct evidence for the upward transfer of energy from the sun's surface toward the corona," says Alan M. Title of the Stanford-Lockheed Institute for Space Research in Palo Alto, Calif. "There is more than enough energy coming up from the loops of the magnetic carpet to heat the corona."

Title's colleague, Mandy Hagenaar of the University of Utrecht in the Netherlands, reported the findings this week at a NASA briefing in Washington, D.C.

Theorist Edward Spiegel of Columbia University notes that the findings "don't solve the problem" of coronal heating completely. The craft can only detect the bundles at the sun's surface, whereas the energy still must travel thousands of kilometers to reach the corona. Spiegel suggests that the energy released by the magnetic field lines gets absorbed by electrons, which transport it upward.

Spiegel is excited that the magnetic bundles appear to be concentrated at the sun's midlatitudes, just where new sunspots occur at the start of each 11-year solar activity cycle. The peak of the next cycle is predicted to occur in about a year. The midlatitudes, enhanced magnetic field density in the could be an early warning sign of that event, Spiegel speculates.