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Frontiers
Sounding Out the Sun's Interior
June 1997
From Earth, it appears that the sun is playing a symphony. Sound waves caused by internal sunquakes roll through the gaseous mass and set the solar surface moving like a drum head.
Solar-physicists are watching the concert with six telescopes set up around the globe. The NSF-funded Global Oscillation Network Group (GONG) cannot hear the sun's music -- "the vacuum in space doesn't transmit the sound" -- but by analyzing the movement of the sun's surface caused by sound waves, researchers are learning about the sun's structure, chemistry and magnetic field.
Gathering solar data is becoming increasingly important as the sun moves into solar maximum, a regular 11-year cycle of heightened solar activity. By the end of the millennium, solar storms are expected to become more common. The storms often affect Earth's man-made satellites and sometimes the planet itself. GONG is helping to predict these storms, but the information has broader implications as well, says project leader John Leibacher, an astronomer at the National Optical Astronomy Observatories in Tucson, Arizona. "The sun is the Rosetta Stone for understanding other stars."
Before GONG, solar-physicists could only study the raging exterior of the sun. They developed theories about the interior but had no way of testing them in detail. By studying the sound waves, researchers are learning that these theories still need work, says Leibacher. "We haven't come up with a new approach, but the old model is wrong."
Among other things, the group found that the convection zone, the region where matter mixes violently, is much bigger than originally thought. Furthermore, it ends abruptly. "There is turbulent mixing and then quiet. We can locate the discontinuity with great precision."
The group has also found clues about the mysterious behavior of sunspots. Sunspots -- places where the sun's magnetic field is very intense -- are associated with solar storms. The dark spots occur near the sun's equator. But so far, no one has known why.
GONG researchers may have found the answer. They discovered that the tumultuous convection zone and the calmer radiative zone rotate at different rates, causing the gases to move in distinctive patterns. At specific spots, some of the gases move up into the convective zone, and some push down into the radiative zone. What's interesting is that gases that move into the radiative zone do so at the same latitude at which the sunspots come up.
GONG researcher Douglas Gough of Cambridge University in England explained his team's theory during the American Astronomy Association's meeting last year. "The idea is that you push all this magnetic field down, and it slowly accumulates until there's enough of it to become buoyant," Gough told The Dallas Morning News. "Then it bursts up and produces a magnetic field which produces a sunspot."
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