THE sun is the nearest and best understood of the stars. Yet it still hides crucial secrets from astronomers. In particular, in spite of 60 years of research, they don't understand the famous 11-year sunspot cycle. And that's a mystery of more than academic interest. Solar activity causes magnetic storms that disrupt communications and power lines on Earth. Flares spew intense streams of protons that endanger spacecraft and astronauts.
One way solar physicists hope to learn more about the activity cycle is to study the inner workings of the sun the way they probe inner workings of Earth, by using seismology. The sun vibrates with very low frequency acoustic waves that, like earthquake waves, provide a way of studying what goes on deep below the surface.
Observers detect the solar vibrations - called acoustic modes - by monitoring surface motions or subtle changes in the sun's overall shape. But, even though they have studied these modes for some 30 years, they have not until recently been able to relate them to the sunspot cycle. Late last month, however, Kenneth G. Libbrecht and Martin F. Woodward of the California Institute of Technology reported in Nature what Dr. Libbrecht calls ``the most accurate measurements of the sun's acoustic modes that have been made so far.''
The data were taken from the Big Bear Solar Observatory near Pasadena, Calif., at the minimum of the current solar cycle in 1986 and at its recent peak in 1988. They show a small but systematic rise in the pitch of solar vibrations of one part in 10,000. The sun seems to change its tune according to the sunspot cycle.
Solar scientists expect to probe such links with the sunspot cycle extensively with a major spacecraft mission supported jointly by the European Space Agency and the United States National Aeronautics and Space Administration (NASA). It's the Solar and Heliospheric Observatory (SOHO). It was to have ridden into space on the shuttle. But, earlier this year, NASA assigned it to an Atlas Centaur rocket. So the current delays in the shuttle schedule shouldn't affect SOHO's planned launch in 1995.
This research is subtle and complex. The sun vibrates at frequencies far below the lowest sound heard by human ears. Moreover, there are at least 10 million different vibrational modes, each with its own unique tone. As SOHO scientist Philip Scherrer of Stanford University notes: ``[The sun is] like a bell in a sandstorm. It rings all its frequencies at once.'' Such is the cacophony in which Dr. Scherrer and his solar seismic colleagues have to find meaningful clues.
The sun is a little like a pan of boiling fudge. Energy from below drives surface activity. Changes in the acoustic vibrations should help in understanding changes in the boiling convection. ``Then,'' Scherrer says, ``we'll understand the forces that are driving the solar cycle. We can't do anything about [solar flares], but we'd like to be able to predict them.''
Space biologists rank such prediction as a top priority for astronaut safety on extended missions. It also should give power companies more timely warning to prepare for dangerous magnetic storms. And better knowledge of such solar processes will help spacecraft designers know how tough to make solar panels and other equipment that is sensitive to solar particle bombardment.
SOHO may not be the most glamorous mission now planned, but it could be one of the most important for spaceflight planners as well as for solar scientists.