Catching waves in the solar wind.

Blinded by the outpouring of light from the surface of the sun, astronomers can't peek directly inside the star, where hydrogen burns to makes helium and a cauldron of hot gases seethes and churns. However, these violent motions hint at internal solar activity by generating vibrations that make the sun ring like a bell. The traveling oscillations, akin to seismic waves that propagate through Earth's crust, cause the sun's surface to rise and fall periodically by small amounts.

By monitoring the solar surface, scientists have tracked some of these oscillations since the 1960s (SN: 4/3/93, p.213). But except for a lone observation that follow-up studies failed to confirm, the tiniest oscillations, which originate in the deepest layers of the sun, have eluded detection. Now, scientists report that they have found strong evidence of this most subtle of vibrations in the solar symphony.

To the astonishment of many scientists, the finding comes from the discovery of tiny, periodic variations in the intensity of the solar wind, the stream of charged particles that blows out from the sun's outer atmosphere. No one had imagined that the wind could retain the pattern of solar vibrations as it travels through turbulent regions of interplanetary space.

The finding may ultimately improve forecasting of solar storms, which can wreak havoc on communication networks on Earth. It might also help explain why the sun emits far fewer neutrinos, a type of subatomic particle, than theory predicts.

Analyzing solar wind data, David J. Thomson, Carol G. Maclennan, and Louis J. Lanzerotti of AT&T Bell Laboratories in Murray Hill, N.J., report their results in the July 13 Nature.

Theory suggests that the sun shows two types of oscillations. The p-mode, or pressure-mode, vibrations move like sound waves and originate as

heat-driven currents just beneath the sun's visible surface. The more elusive g mode, or gravity mode, resembles the bobbing up and down of water waves. It's these vibrations, generated when a low-density gas pushes against a high-density gas closer to the core of the sun, that the scientists believe they have now detected.

The researchers weren't looking for waves when they began analyzing the intensity of the hydrogen and helium ions in the solar wind recorded by the Ulysses craft between 1992 and 1994. Rather, they were hoping to find a correlation between solar activity and the failure of several communications satellites. To their surprise, the data revealed that the intensity varied at two characteristic sets of time intervals--one about every 5 minutes and the other ranging from several hours to several days.

The 5-minute variation corresponds to the well-known p mode; the longer variations seemed to represent the long-sought g mode. Doubting their own result, the researchers turned to similar data, taken in 1985 by the Voyager 2 craft. They found the same g mode pattern.

The scientists speculate that the magnetic fields at the sun's surface sense the g-mode vibrations and communicate them to solar wind ions that travel with the fields. Supporting that argument, an analysis of data collected by other spacecraft from 1977 to 1981 shows that the magnetic field oscillates in synchrony with the g-mode waves. Nonetheless, says Thomson, "we've uncovered more puzzles than solutions."

The findings offer both good news and bad news about the troublesome effects of the solar wind on Earth, the team notes. The wind may have stronger surges than expected, but their periodicity makes them more predictable.

Discovery of the g mode may bring predictions about the emission of solar neutrinos more in line with observations, Thomson says. In addition, these waves may cause gases at the sun's core to mix, which could alter neutrino production.