December 19, 1997
Their preferred hypothesis postulates a region of intense magnetic fields rotating deep within the solar interior. Because neutrinos are created in the nuclear reactions that take place at the Sun's core, such a region might disrupt the flow of neutrinos to Earth. For this hypothesis to hold, however, the basic properties of the neutrino must be redefined in a way that conflicts with the "standard model," the well-tested description of the nature of fundamental particles and forces. Theories that redefine the neutrino in this fashion have been advanced to explain the missing mass in the universe and the unexpectedly low number of neutrinos that have been detected coming from the Sun.
An alternate explanation might be regular pulsing in the strength of the nuclear reactions themselves. There is no outward evidence for such a cycle. But light created at the Sun's core takes 100,000 years or more to reach the surface, so any fluctuations in the production of radiation in the core could be completely smoothed out by the time the light reaches the Sun's surface, the researchers propose.
The statistical evidence for this cycle is reported in the Dec. 10 issue of the Astrophysical Journal by Peter Sturrock, professor of applied physics; Guenther Walther, assistant professor of statistics; and physics research associate Michael Wheatland. Their analysis is based on data collected at the Homestake neutrino detector in South Dakota over a 24-year period. Using advanced statistical procedures, they found clear evidence for a 28.4-day cycle. "We estimate the probability that the cycle is due to chance to be about three parts in a hundred," Walther said.
Despite their ghostly nature, neutrinos can be detected because they do interact occasionally with ordinary atoms and molecules. In one such interaction, neutrinos change chlorine atoms into argon. Despite the enormous number of neutrinos passing through the Earth, this interaction is so rare that just to detect it scientists have been forced to build tanks holding tens of thousands of gallons of chlorine-containing liquid and develop methods for picking individual argon atoms out of such large volumes of liquid. It wasn't enough to do this on the Earth's surface, either, because cosmic rays also can convert chlorine into argon. So the tanks had to be buried deep underground.
The first successful solar neutrino detector was located in the Homestake Gold Mine. Buried a mile underground, the detector contains 100,000 gallons of carbon tetrachloride. For the last 30 years, the Pennsylvania State University scientists who operate the detector have filtered out and counted the argon atoms that have accumulated every few months. Over this period, the instrument has identified an average of about one neutrino event every two days.
That rate is about one-third the number that scientists who study the solar nuclear reactions had predicted. Two other neutrino detectors, the Kamiokande experiment in the Japan Alps and the Gallex experiment in the Gran Sasso Laboratory in Italy, both have verified the shortfall measured at Homestake.
One possible explanation for this deficit was that the neutrino-producing nuclear reactions were happening more slowly than the scientists expected, which would be the case if the temperature at the Sun's core were about one million degrees Celsius less than predicted. But observations of sound waves traveling deep into the solar interior have provided a temperature measurement of 15.6 million degrees Celsius, too hot to lower neutrino production.
The situation has led some scientists to invoke "new physics" to explain the low observed numbers. According to standard nuclear physics, a neutrino at rest does not have any mass. Now some theoretical physicists are proposing that these particles may have an infinitesimal but non-zero mass. Several major experiments have been built to test this proposal. Sturrock and his colleagues use these new theories to explain the variations in neutrino flow that they have found.
If neutrinos have any mass at all, they would help account for the "missing matter" in the universe. Astronomers have found that galaxies act as if they are swirling around a center of mass substantially larger than scientists can account for by summing up the amount of visible matter that they contain. Neutrinos with mass could account for at least some of this "dark matter."
Directly measuring this effect is the purpose of the Palo Verde Neutrino Oscillation Project, headed by Stanford Associate Physics Professor Giorgio Gratta and Professor Emeritus Felix Boehm from Caltech. They led the design and construction of a neutrino detector a mile from the Palo Verde Generating Station in Arizona to determine if the neutrinos produced by the station's nuclear reactors undergo this cycling effect.
This effect could explain the shortfall in solar neutrinos. Only one of the three types of neutrino, the electron neutrino, is detectable. If the electron neutrinos produced by the Sun change into muon and tau neutrinos en route, it would mean that only one-third of the neutrinos reaching Earth would be detectable.
To look for regular variations in the number of neutrinos reaching Earth, Sturrock and his colleagues analyzed the data collected at Homestake. Because the data were collected about four times a year, it normally would be impossible to use this information to identify cycles as short as 28 days. But the data were not collected at regular intervals. That allowed the researchers to piece together evidence for a shorter cycle by constructing a detailed computer simulation of the detector, running thousands of simulations, and comparing the outcomes with the detector's actual observations.
In an earlier analysis, conducted in 1995 and 1996, the researchers thought they had found evidence for a 21.3-day peak. This was reported in the News and Comment section of Science magazine. When they submitted this analysis to a scientific journal, however, one of the reviewers was unable to duplicate their results. When the researchers reworked the problem from scratch, they discovered an error in their transcription of the Homestake data. When this was corrected the 21.3-day cycle disappeared.
The researchers find the 28.4-day cycle particularly intriguing because it corresponds almost exactly to the rotation rate of the Sun's interior, as seen from Earth. The Sun is made up of three parts: the core, where the nuclear fusion reactions that power the Sun take place; the radiative zone where energy is transported outward from the core; and the outer, convective zone. The radiative zone rotates as if it were a solid, rather than a gaseous body, at this same rate. So Sturrock and his collaborators speculate that the source for this cycle in neutrino flux may originate in the radiative zone. A region of extra-intense magnetic fields might modulate the flow of these particles, they suggest.
Sturrock and his co-authors invoke this effect as their preferred explanation for the cycle that they have found. Neutrinos spin in either a left-handed or right-handed direction. Nuclear reactions produce only left-handed neutrinos, and only left-handed neutrinos take part in nuclear reactions such as converting chlorine into argon. If different parts of the Sun have different strength magnetic fields, the flux of left-handed neutrinos will vary as they travel in different directions from the Sun. That would lead to a detection rate on Earth that varies with the Sun's rotation period.
As the Earth orbits the Sun, the neutrinos that are detected on Earth pass through different solar latitudes, due to a tilt in the Sun's axis of rotation. This can produce other, weaker cycles centered on the basic rotation frequency. The period of 28.4 days corresponds to a frequency of 12.9 cycles per year. Sturrock and his collaborators have also found evidence of the expected "sidebands" at 10.9, 11.9, 13.9 and 14.9 cycles per year.
At the same time, the new analysis did not find any evidence for neutrino variations that correspond to the 11-year solar cycle and only weak evidence for two other proposed cycles: a 157-day periodicity that Eric Rieger of the Max Planck Institute in Germany found in the intensity of solar flares, and a 780-day "quasi-biennial" periodicity that Kunitomo Sakurai from Kanagawa University reported finding in the Homestake data.
"I thought [Sturrock and his colleagues'] analysis was quite convincing," said Jeffrey Scargle, a research astrophysicist at NASA's Ames Research Center, who is an expert in this type of analysis. "The method they used was very good and they made a really good case for the signal being in the data. Of course, what this means for solar and particle physics is quite problematic."