Revealing the sun's solitary vibrations.

The sun rises, its face warm, placid, reassuring. But on closer inspection, our home star actually behaves like a bubbling cauldron, with heat-driven convection currents setting vibrations ringing throughout its interior.

Astronomers began tuning in to these vibrations in the 1970s and have since learned a great deal about the sun's dynamics (SN: 7/2/88, p.8). Now, researchers have hit upon a new way of extracting information from this solar symphony The technique, developed by NASA solar physicist Thomas L. Duvall Jr. and his colleagues at the National Solar Observatory (NSO) in Tucson, Ariz., may enable astronomers to use individual acoustic waves traveling through the sun to study its interior, much as seismologists use the echoes of earthquakes to probe geologic structures deep within the planet.

"This really brings us a little closer to what's being done in terrestrial seismology," says Duvall. The researchers report their findings in the April 1 NATURE.

Usually helioseismologists observe the entire spectrum of solar vibrations simultaneously The sun, they have discovered, resonates most strongly at specific frequencies, or pitches, much like an enormous pipe organ. The new technique enables researchers to track the motion of individual acoustic waves, which emanate from a relatively thin, active layer near- the sun's surface.

These waves travel downward into the sun, where changes in density bend them back up toward the surface. The waves deflect off the underside of the surface, creating ripples that astronomers can observe from Earth. Each wave can repeat this process 50 or more times.

"This is a major advance;' declares John W. Leibacher, director of the NSO and an early practitioner of helioseismology The technique, says Leibacher, may help researchers probe smaller regions of the sun, perhaps illuminating the mysterious subsolarian processes that generate sunspots.

However, the proof of the new method will be in the doing, comments solar physicist Timothy M. Brown of the National Center for Atmospheric Research in Boulder, Colo. As it stands, the technique may prove somewhat insensitive to low-frequency vibrations, Brown says. This may hobble efforts to chart relatively slow, albeit massive, flows of material far below the sun's surface, he explains. Nonetheless, he adds, the technique represents "an important move in the right direction."