LOS ALAMOS, N.M., May 31, 1996 -- Solar physicists from Los Alamos National Laboratory and other institutions have turned into seismologists to advance their understanding of our nearest star.
Just as geologists study seismic waves from earthquakes to decipher Earth's structure, astronomers now study sound waves traveling through the roiling surface gases and dense inner regions of the sun to probe its structure, using this important new observational evidence to evaluate and improve long-standing theoretical models of the sun.
A number of papers in today's (May 31) issue of Science magazine, including two with authors from Los Alamos, describe the new observational evidence and its implication for our understanding of the sun.
"Our solar models agree with the new data to within a few percent, which is very good," said Joyce Guzik, Los Alamos solar researcher and co-author on two Science papers. "The problem is the GONG data are so accurate that we can see these small differences and now we have to try to figure out what in our models is not correct. Even though these are little differences, they may be the clue to something big."
GONG stands for Global Oscillation Network Group, an array of six identical instruments spread around the globe that provide essentially continuous observations of the sun. The instruments measure the rise or fall of material on the sun's surface and thus can detect the passage of an acoustic wave as it displaces material.
Such waves are generated in the sun's turbulent outer layers, like the thundering sound felt by someone standing near a large waterfall. The solar waves travel inward and are reflected back to the surface; the depth to which they travel depends on the nature of the waves themselves and the sun's internal structure. Thus, by analyzing the myriad waves that appear and dissipate across the sun, researchers can gather information about virtually any point of the sun's interior and can, for the first time, confirm the details of theoretical models about the sun -- and raise some interesting problems for further consideration.
"These data and the constraints they put on our solar models help us better understand the sun, how the solar system came to be and what its ultimate fate will be," Guzik said. "In addition, we can learn about the physics of dense plasmas by studying conditions we can't create in the laboratory or ever get to on Earth." The only situation that comes close to matching the temperatures and pressures of the sun's interior, Guzik said, is the detonation of a nuclear weapon.
One issue the GONG data help address is known as the solar neutrino problem, in which only about one third the number of neutrinos, an energetic nuclear particle released during fusion reactions, expected from the sun are detected. Some researchers thought the solar models were not accurately representing the sun's central conditions and thus overestimating the number of neutrinos the sun was emitting. The GONG data show the models are highly accurate and that the answer to the neutrino problem lies elsewhere, probably in the nature of the neutrino itself.
Guzik said some of the helioseismic data suggest that the sun's core spins at about the same rate as its surface. Astronomers have long expected that the sun's inner, denser regions would spin faster than its surface, just as an ice skater's spin speeds up as he pulls in his arms and concentrates his mass toward his spin axis.
"If the core rotates at the same rate as the surface, that's a problem," Guzik said. "You have to explain where the core's angular momentum went, and the problem has implications for how the solar system formed."
Likewise, a rapidly spinning core would help explain an absence of the element lithium in the sun's outer layers. The lithium present in the sun's outer layers when it formed should mostly still be there, unless it was dragged into the sun's interior where conditions are extreme enough to burn up the lithium. A fast-spinning core could have dragged the lithium to destruction depths, but without it the problem lingers unexplained. That points Guzik and other theorists to alternative considerations. For example, a strong solar wind that carried away a tenth of the sun's original mass, leaving it in its present state, could also explain the paucity of lithium. Models that include mass loss, in fact, agree better with the GONG observations.
Already the GONG data have inspired several changes to solar models, Guzik said, including improvements to the models' opacities (a parameter that accounts for the interaction of matter and photons) and the nuclear reaction rates used in the computer codes, and the recognition that it is important to include in the models the settling of helium and heavier elements at the base of the sun's upper, convective layer.
Guzik, who is a member of the GONG solar modeling team, works in Los Alamos' Applied Theoretical and Computational Physics Division. She is currently developing a two-dimensional, rotating evolutionary model of the sun -- using a code developed by Robert Deupree of Los Alamos' Dynamic Experimentation Division -- that will allow her to address some of the outstanding questions. Guzik also is conducting asteroseismology on delta-Scuti stars, a class of stars that has many fewer oscillations than the sun does. Her work is supported by a Laboratory Directed Research and Development grant and by a grant from NASA.
Guzik and Art Cox, longtime staff member, fellow and currently an affiliate with the Laboratory, were co-authors on the paper "The Current State of Solar Modeling" in the May 31 Science; the lead author was J. Christensen-Dalsgaard from Aarhus University in Denmark. Guzik was also a co-author on "The Seismic Structure of the Sun," a paper led by Douglas Gough of the University of Cambridge.