Seismic Waves Used to Investigate Sun’s Interior

Astronomers are using a new technique similar to that being used by geologists studying earthquakes to learn about the interior of the sun, scientists said here Sunday at a meeting of the American Assn. for the Advancement of Science.

The new technique, called helioseismology, has already provided surprising information about the sun’s rotation and new data about the composition and temperature of the interior, said astrophysicist Robert W. Noyes of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Scientists are interested in learning more about the nuclear fusion that occurs in the sun’s interior because they hope that controlled nuclear fusion will become an important power source on earth.

Seismology is the analysis of sound waves that pass through the Earth’s interior. The speed of the waves depends on the type of rock the waves pass through, so analysis gives information about the composition of the earth.

Seismic waves on Earth are generally initiated by earthquakes or nuclear explosions, although geologists searching for oil and other minerals set off small explosions to map limited areas. In contrast, seismic waves in the sun are generated continuously by the motion of gases “and are always present at all points within the sun and on its surface,” Noyes said.

“The sun is ‘ringing’ like a bell, but not like one that is struck by a clapper; rather it is vibrating somewhat like a bell suspended in a sandstorm continuously struck by tiny grains of sand.”

“The most exciting aspect of helioseismology is that, for the first time, it actually allows us to see what is going on inside the sun,” said astronomer Jay M. Pasachoff of Williams College-Hopkins Observatory in Williamstown, Mass.

Small Segments Vibrate Up and Down

Noyes first discovered these seismic vibrations in 1963 when he was a graduate student at Caltech. He observed that small segments of the sun vibrated up and down over distances of about 25 miles with a period of about five minutes.

He and others have since found that these large vibrations are produced by as many as a million much smaller vibrations. These individual vibrations can be discerned with sophisticated instruments, but the process requires continuous observation of the sun over periods of many days.

“This is somewhat difficult to do in a conventional observatory where you normally get only eight-hour periods of light interrupted by 16-hour periods of darkness,” Noyes said. To overcome this problem, some scientists have gone to the South Pole during the austral summer to continuously monitor the sun.

The United States is planning to establish a network of six monitoring stations around the world in order to measure the solar oscillations continuously. The program will cost $10 million, he said.

One thing that has already been learned from helioseismology is that the interior of the sun is not rotating as fast as had been believed. The surface of the sun rotates once about every 25 days. But since the sun is entirely gaseous, it does not all rotate at the same speed.

Observations of younger stars with the same mass as the sun suggested that the interior of the sun should have a rotational period of only a few days, Noyes said. If that were the case, theoretician Robert Dicke of Princeton has observed, it would change the sun’s gravitational pull in a manner that would account for certain eccentricities in the orbit of the planet Mercury.

Those eccentricities are now explained by Einstein’s theory of general relativity. Scientists argued that if they were instead explained by the rotation of the sun’s interior, Einstein’s theory would be cast in doubt.

Noyes has found, however, that the interior of the sun rotates at roughly the same speed as its surface. “Thus it appears that Einstein’s theory is safe,” he said.

Unfortunately, helioseismology has provided little information about one of the most nagging problems of solar science--the so-called missing neutrino problem. Other scientists here noted that foreign scientists are initiating major new programs to attempt to solve the missing neutrino problem.

Those programs are based on ideas put forth by U.S. scientists, but will have little or no U.S. participation because of lack of funds. The U.S. failure to fund similar projects, said physicist John N. Bahcall of the Institute for Advanced Study at Princeton University, is “a national disgrace.”

The missing neutrino problem arises from the work of physicist Ray Davis of the University of Pennsylvania. Since 1967, Davis has been operating a neutrino detector--essentially a 100,000-gallon swimming pool filled with dry-cleaning fluid--in a mine deep under South Dakota.

Such a massive detector is required because neutrinos, elementary particles that travel at the speed of light, have virtually no mass and thus have a very low probability of bumping into molecules in the detector. Large numbers of molecules must thus be used to increase the likelihood of a collision that can be observed.

Davis has seen only about one-third as many neutrinos as are predicted by theory. Therefore, either the theory is wrong and must be changed or there is something between the sun and the Earth that is absorbing the neutrinos or changing them so they are not detected.

To solve this problem, Davis proposed a second detector, based on 50 tons of pure gallium, to detect neutrinos with lower energies. Such neutrinos are produced in much larger quantities in the sun, Bahcall said, and the theory behind their formation has a much firmer foundation. Observing them should show for certain whether something is absorbing neutrinos.

Unfortunately, Bahcall noted, Davis’s grant request was turned down.