Link Found Between Neutrino Flux and Solar Wind

Scientists have long been perplexed by the so-called solar neutrino problem. That is, the apparent inconsistency between the theoretical prediction for the flux of solar neutrinos and the actual flux detected by various Earth-based experiments set up around the world. Meanwhile, many other solar parameters, including the power output of the Sun, have remained steadfastly true to the tenets of the standard solar model.

The quandary has called into question scientists' fundamental understanding of how stars work and what neutrinos - these elementary chargeless particles produced by nuclear fusion reactions in the Sun's core - really are. Now a paper to be presented at AGU's Spring Meeting in Baltimore is reigniting the controversy over the underpinnings of the standard model of particle physics.

Every second, according to models of the Sun's interior, tens of billions of neutrinos pass through every square centimeter of the Earth. While en route to Earth, the ephemeral particles neither age nor have time to do anything but remain neutrinos, explains solar physicist Gordon Emslie, of the University of Alabama at Huntsville. Why? The model holds that neutrinos are massless particles that consequently travel at the speed of light. Once the neutrinos are emitted, Emslie tells, they must be detectable because they cannot decay.

Neutrino counting rates have remained low, nonetheless. The long-term averaged neutrino flux measured at the four established spectroscopic detection sites has consistently been lower than that predicted by the standard solar model. While the model predicts one neutrino a day, the detectors collectively pick up about one neutrino every three days, as the neutrinos interact extremely infrequently with matter.

For example, the oldest detector at the Homestake gold mine in South Dakota has actually recorded only about one-third of the predicted number of solar neutrinos. The Homestake experiment dates to the 1960s when it was built by Ray Davis, then at Brookhaven National Laboratory. The experiment is essentially a tank filled with millions of gallons chlorine-37 bleach that capitalizes on the slight interactions between neutrinos and some isotopes. When the chlorine atoms absorb a solar neutrino, a neutron is converted into a proton yielding, in this case, radioactive argon gas, which then can be detected when the argon gas atoms decay.

If neutrinos possess a minuscule amount of mass, however, the big picture may well be different. Even a mass of one ten-thousandth of an electron would enable neutrinos to decay when they travel from the Sun to the Earth. Such a finding of even this tiny mass would also contradict the standard model, Emslie says. Some recent laboratory experiments involving radioactive decay suggest that neutrinos may indeed have a tiny mass. Ironically, neutrinos were first predicted in the early 1930s to have zero mass on the basis of other radioactive decay experiments.

In other words, to account for the observed discrepancy between the model and the measurements, scientists have suggested that if the neutrinos have a finite mass, they may "oscillate" into a neutrino species that the experiments cannot detect. Various teams have suggested that such a variation - besides being the stuff nobel prizes are made of - may be anticorrelated with sunspot number. However, direct evidence of this oscillation has remained elusive.

Yet some scientists have proposed what many consider to be a far more controversial possibility to explain away this conundrum: the existence of a time variation in the measured neutrino flux at the Earth. Now Ralph L. McNutt Jr. of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., may have deciphered another dimension to the puzzle. In a paper to be presented at AGU's Spring Meeting in Baltimore, McNutt claims to have found a variation in the solar neutrino flux on a timescale of about a decade. What's more, the variations in neutrino flux are correlated with variation in the strength of the solar wind, as determined by observations from the IMP-8 spacecraft, which has been observing the solar wind for nearly 20 years - 16 of which coincide with the operation of the Homestake neutrino experiment. Like the solar wind, McNutt now says, the neutrino flux has an 11-year solar cycle. The solar wind is strong when the Sun's magnetic field "opens up" into space, allowing particles to escape. Later in the cycle, on the other hand, the field is held tightly to the Sun, the solar wind fades away and is replaced by an increased number of sunspots and solar flares. These phenomena are related to an increase in the amount of "closed" magnetic field on the Sun, Emslie explains.

In McNutt's new analysis, the correlations with neutrino capture flux are most pronounced for particle flux, followed by density and momentum flux. McNutt found no correlation, however, between solar wind and neutrino speed. This indicates that the number of particles in the solar wind - and not how fast they are emitted from the Sun may be key to the relation.

Raju Raghavan, a particle physicist and astrophysicist with AT&T Bell Laboratories in Murray Hill, N.J., called the finding, "interesting, but not yet exciting."

McNutt's results are consistent with earlier work showing that the neutrino flux tends to be high when the sunspot number is low, and vice versa. In fact, McNutt claims these three new correlations are significantly better than the anticorrelation found by applying the same technique to daily values of the sunspot numbers.

Exactly how the neutrino flux variations are produced remains unclear. At any rate, the correlations provide new evidence that the neutrino flux does vary. If the flux is modulated by changes in the Sun's magnetic field, the neutrinos must somehow decay, Emslie says, which implies that they must have a nonzero mass.

Yet AT&T's Raghavan says the finding does not necessarily mean neutrinos have nonstandard properties. Despite that "any evidence of correlation is definitely useful," Raghavan says the finding neither implies a low neutrino mass nor directly links the modulation to the Sun's magnetic field. To prove a direct correlation with the magnetic data, Raghavan says, neutrinos would have to be shown to have a magnetic moment and more would have to be known about the magnetic field of the Sun. But for now, scientists don't know the depth or the extent of the solar magnetic field nor how it changes with sunspots, Raghavan says.

In addition, Emslie says there may be some limitations to the solar wind measurements. "Solar wind flux is not constant in all directions. Therefore the variations in the solar wind flux at IMP-8 are not necessarily representative of the flux as a whole."

Yet if confirmed, this result would bring into question significant pieces of the standard solar model or the standard particle physics models or perhaps both. Yet McNutt thinks the evidence is stronger than that. He claims the findings not only imply but "require new fundamental physics beyond the standard model of electroweak interactions."

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