New observations of sun's interior shed light on magnetic field activity

March 29, 2000

Scientists have detected changes in the rotation rates of violent, charged gases some 130,000 miles beneath the sun's surface, a finding that may help them better understand the physical dynamics of the 11-year solar cycle that affects Earth.

University of Colorado at Boulder Professor Juri Toomre said the surprising discovery indicates there are significant speed-ups and slow-downs in the rotation rate of gases at the inner edge of a spherical shell known as the convection zone that makes up the outer 30 percent of the sun's radius.

The largest changes in the rotation rate occur at the interface between the convection zone and the adjacent radiative interior 130,000 miles into the sun's interior at a place called the tachocline.

The tachocline is important because it is believed to be the place where the "solar dynamo" is operating, said Toomre, a professor in CU-Boulder's astrophysical and planetary sciences department. "As a physical process, the dynamo is essentially a great factory that builds and regulates the magnetic field of the sun. This is the first time we have been able to measure changes in the heart of the solar dynamo."

A paper on the subject by Rachel Howe of Tucson's National Solar Observatory, Toomre, and researchers at Stanford University and institutes in England and Denmark was published in the March 31 issue of Science.

The data for the discoveries came in part from the Solar and Heliospheric Observatory, a joint spacecraft of NASA and the European Space Agency that is positioned toward the sun about 1 million miles from Earth. Information also was obtained using the Global Oscillation Network Group, or GONG, a series of ground-based observatories around the world that monitor solar activity 24 hours a day. Toomre is the head of GONG's Scientific Advisory Committee.

Both SOHO and GONG analyze motions at the surface of the sun caused by sound waves reverberating through the sun's interior. The spacecraft and ground observatories measure up to 1 million specific locations and their corresponding sound pulses.

"We have to listen to the sun for a few months at a time to detect the large-scale motions that carry the different sound waves around," said Toomre.

Using data from observations over the past four years, the scientists found "unexpected changes" in the rotation rate at the tachocline, showing it took about 15 months to rotate near the equator and about 12 months at higher latitudes, said Toomre. At some phases, the rotation rate in the tachocline appears to speed up at the lower edge of the convection zone and simultaneously slow down in the adjacent edge of the radiative zone located closer to the sun's interior, he said.

"This is the first indication there are speed-ups and slow-downs in the area the solar dynamo is thought to be operating," said Toomre. We hope to be able to model the activity of the solar dynamo, which is linked with the 11-year solar cycle."

The peak of the 11-year cycle is marked by magnetic fields breaking out on the sun's surface as sunspots, solar flares and coronal mass ejections spewing hundreds of thousands of miles into space, he said. These outbursts are capable of disrupting satellite-based communications and power grids on Earth, he said.

"One of the great puzzles in nature has been why the sun has the 11-year cycles of magnetic activity," said Toomre, also a fellow of JILA, a joint institute of CU and the National Institute of Standards and Technology. "The solar dynamo is responsible for the extremely violent but orderly rythym in the cycle, including the stretching and breaking of magnetic field strands that eventually burst out onto the sun's surface."

Toomre and his CU-Boulder colleagues use supercomputer computations to construct three-dimensional, high-resolution models of the complex fluid dynamics related to solar processes. Such models help to illuminate the interaction of small-scale and large-scale features on the sun.

The sun currently is very close to its 11-year peak in magnetic activity, which will occur in the next one to two years, and already is displaying large sunspots, solar flares and coronal mass ejections, he said.
Images are available on the web at: PR/SSU_2000/Tachocline/ .

University of Colorado at Boulder

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