Making sunshine; new observations focus attention on the sun's missing neutrinos

Science News, Oct 28, 1989 by Ivars Peterson

Solar scientists use what they call the standard solar model to account for the sun's behavior and to calculate the expected neutrino flux. In the latest version of the model, Bahcall and Roger K. Ulrich of the University of California, Los Angeles, start with the assumption that the primordial sun was spherical and had an even distribution of chemical elements. They deduce the abundance of heavy elements from recent observations of the solar surface, but allow the ratio of hydrogen to helium to vary. Trying various initial conditions, they calculate how their model sun evolves.

New knowledge leads to refinements of the model. "The solar model has been changed every year or two for the last 25 years that I've been working on it," Bahcall says. "But the changes have not been large."

To account for the observed solar-neutrino deficit, theorists have proposed a number of "nonstandard" solar models. For example, the sun may have a different composition, internal temperature or pattern of convection currents than assumed in the standard model. Any of these factors could reduce the expected neutrino flux.

Indeed, both the Kamiokande and chlorine detectors are sensitive only to a small portion of the neutrinos produced by the sun. They pick up mainly the high-energy neutrinos resulting from the nuclear reaction producing boron 8. The rate of that reaction happens to be particularly sensitive to the sun's temperature at its core.

Helioseismology -- observations of the way the sun's surface rises and falls in a pattern of oscillations -- provides one of the best checks on these nonstandard models by supplying information about the sun's internal structure, for example, how deep the sun's convective zone goes. Solar-oscillation data have already ruled out several proposed nonstandard solar models. As these data get better -- particularly for conditions near the sun's core, where the greatest uncertainties lie -- more nonstandard models are likely to fall.

On the other hand, the standard solar model seems to fit the oscillation data well. "Preliminary indications are that the standard solar model, to the accuracy to which we need it for predicting neutrino fluxes, has been verified as far as helioseismology can presently go," Bahcall says.

One initially promising idea that no longer looks so attractive is the possibility of the existence of previously undetected particles known as weakly interacting massive particles, or WIMPs. Particles in this class would presumably accumulate at the sun's center, slowing the nuclear reactions and lowering the sun's central temperature. But solar-oscillation data don't support the idea that the solar interior is cooler than postulated in the standard solar model.

"The idea of weakly interacting particles in the more general context of [the invisible] dark matter in the universe is still a ver exciting possibility," Wolfenstein says, "but the particular application to the sun seems very artificial." However, he cautions, "our job is to find out what the world is like, not what we want it to be like."