Blueprint of a Star

A million miles from the earth, the Solar and Heliospheric Observatory (SOHO) stares unblinking at the sun. Launched in December 1995 as a $1.2 billion joint mission of NASA and the European Space Agency, SOHO hangs at the point where the gravitational tugs of the earth and the sun are in balance. Because it orbits the sun in step with the earth, the observatory’s visual and ultraviolet cameras and spectrometers monitor the star continuously. Now a crescendo of observations, gathered over the first two years of the mission, is leading scientists to a new understanding of the fusion-energy furnace whose light and warmth make life on earth possible.

Included among the data are findings that space scientists say are quite startling. Peering beneath the sun’s blinding brilliance, they have observed for the first time features whose presence they had only hypothesized before. These new observations, in turn, are helping to solve at least one long-standing mystery: how the sun’s corona is reheated.

Other solar-observing instruments orbit the earth (so the planet periodically blocks their view of the sun) or are earthbound and thus hampered by clouds and the atmosphere’s absorption of ultraviolet and infrared radiation. SOHO’s uninterrupted view and its array of advanced instruments have allowed scientists to observe previously unrecognized features of the sun’s architecture. By measuring Doppler shifts in the light emanating from the sun, for example, scientists are able to analyze motions on the sun’s surface and make inferences about its underlying structure, much as seismologists on earth measure activity on the planet’s crust to learn about its interior.

“We’re seeing what the internal structure of the sun is,” says Arthur Poland of NASA’s Goddard Space Flight Center, the space agency’s chief scientist on the project. “It’s giving us an understanding of how stars work. What we learn would be a blueprint for all stars.”

Scientists have long known that the sun, like the earth, has an unmoving inner core and a rotating outer core 130,000 miles deep, known as the convection zone. Computer models had predicted turbulence in the convection zone. But now scientists are discovering that activity in the convection zone is even more complex than they had anticipated.

“What we see through helioseismology is a convection zone that is interesting and bizarre,” says Richard Canfield, a physicist at Montana State University at Bozeman.

For example, scientists had previously observed movement of gases on the surface of the sun, but SOHO’s data show that the currents run much deeper than they expected: the entire outer layer of the sun-to a depth of 15,000 miles-is flowing slowly from the solar equator to the poles. In addition, SOHO has discerned great “rivers” of gas circling beneath the poles in a pattern similar to the currents of the earth’s jet stream. These were “a complete surprise,” Poland notes.

In addition, SOHO’s instruments have allowed scientists to witness shock waves rippling across the solar corona as gases explode above the sun’s surface and fall back. Although some theoreticians had predicted these waves might occur, Poland says, they had never before been seen. “It was a Wow, that really does happen!’” he adds. “The theoretician might say, I told you so!’”

Scientists are also using SOHO to study the solar wind, the continuous stream of proton and electron particles that blow off the sun and flow through the solar system. Using a coronagraph to block the ball of the sun and sensitive digital cameras equipped with charge-coupling devices to view its edges, they have discovered that powerful currents or eddies that emanate from the sun’s equators play a more important role in distributing the solar wind than they had realized. And they have detected previously unknown elements in the solar wind, adding nickel, iron, silicon, sulfur, calcium and chromium to the list of 11 or so known elements.

“The quality of these telescopes is so good that you can actually watch material boiling off the sun in the solar wind,” Canfield says.

SOHO appears to have solved one key riddle confronting researchers: how the solar corona gets so hot. The surface of the sun is 5,000 degrees Kelvin, yet the gaseous halo beyond, called the corona, reheats to 1 million degrees Kelvin. “A major question regarding the sun is why the corona is so hot,” Canfield says.

The answer seems to lie in pancake-shaped regions of hydrogen and helium that Poland found moving upward through the sun. These pancakes arise from friction between the sun’s inner core and its outer core. Their upward movement, he says, releases kinetic energy that is transformed into magnetic energy and radiates outward from the sun’s surface.

Scientists had assumed that these regions of gas were spherical, like the bubbles formed in boiling water. Computer models based on this assumption, however, failed to explain the heating of the corona. The counterintuitive flat, oval shapes fit the model much better, Poland says, and their discovery may finally give scientists an accurate understanding of the process.

“We had no way of looking at these before,” he adds. “It’s like Galileo looking for the first time. We have to reconsider our view of how the sun works.”