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Science & Technology

Solar heartbeat

For the record
Omer Manya
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From Diana J. Choyce
Apr 10 - 16, 2000

Layers of gas circulate beneath the sun's surface

Solar Astronomy is the study of the solar cycle. For some this would seem a rather boring pursuit. But for Astronomers from the National Science Foundation's National Solar Observatory (NSO), it has provided a wealth of information about our Sun. Recently they have discovered a solar "heartbeat" in the layers of gas that circulate beneath the sun's surface. Their studies have shown that about ever 16 months, some layers actually slow down or speed up. This knowledge should help explain the cycle of activity that is observed on the sun's surface. Though usually quiet, every 11 years the sun produces a high level of sunspot and solar flare activity. These eruptions can cause interference in the use of cellular phones, power distribution systems, and satellites. Rachel Howe, Frank Hill and Rudi Komm of the NSO in Tucson, Ariz., and their colleagues analyzed more than four years of observations from the Global Oscillation Network Group (GONG), a worldwide network of solar telescopes, to detect and model motion inside the sun. They reported their results in the March 31 issue of Science.

The sun is made up of many layers of gas which scientists probe to analyze sound waves traveling through the sun's core. The method used is called helioseismic, and is similar to methods used to study earthquakes. Howe's team examined layers extending almost halfway to the solar core and measured the speed of movement at different depths. They believe the patterns in these movements are connected to the cycle of eruptions seen on the surface. "We listen to the sun's heartbeat to understand what is happening in its core," explains Hill. Unlike the earth, all points on the solar surface do not rotate at the same rate. The solar equator rotates once every 27 days, while the rotation rate at the sun's poles slows to once every 35 days. This "differential" rotation, long a mystery of solar physics, extends through the sun's turbulent convective layer, located about 210,000 kilometers below the surface — nearly one-third of the distance to the solar core. Below this layer, the differential rotation vanishes.

At the edge of the convective layer, Howe and her colleagues used GONG data to determine that the rotation rate varies periodically, completing a cycle about every 15-16 months. The team used data from the NASA and European Space Agency's Solar and Heliospheric Observatory (SOHO) spacecraft to confirm the pattern of these variations. "At first we were skeptical of the pattern. Knowing the complexity of models used to explain the solar magnetic field and its connection to observed solar activity, we were expecting nothing, or chaos, in our observations at that location," said Howe. The GONG network (http://www.gong.noao.edu/sites/sites.html) is an international project led by the National Science Foundation. It provides continuous observations of the sun, monitoring the surface and tracking its tiny oscillations 24 hours a day. These oscillations are visual evidence of the sound waves traveling through the sun's interior.

"We don't yet understand exactly how the cycles are related to sunspot activity, but because they are taking place in the tachocline, which is believed to be where the sunspot cycle originates, there must be a connection somewhere," Howe said. "It's one more piece of a puzzle that theorists may one day be able to fit together to explain the solar cycle." The new findings show that just above and below the tachocline, and close to the sun's equatorial plane, the rotation rate speeds up and slows down rhythmically every 16 months or so, Howe told SPACE.com. "When the material above the tachocline is moving faster than average, the material below is moving more slowly, and vice versa."

The study also raised the question of whether there is a link between these observed deep changes and bands of gas nearer the surface. The surface "rivers" of gas that move parallel to the equator move slightly faster or slower than the average speed of the surrounding gas. Although the effect is subtle, it is persistent. Bands of fast and slow gas gradually move from high latitudes towards the equator over the years. Simultaneously, sunspots are sighted closer and closer to the equator as their 11-year cycle approaches its maximum of activity. The findings might lead to a better understanding of the chemistry of the convective zone, but the researchers caution that this part of the work is speculative. The pulsing rotation rates might mean that material in the sun's relatively distinct layers is being mixed. There is less lithium at the sun's surface, for example, than researchers would expect. "That might be explained if the lithium in the convection zone is being mixed down into the deeper layers, where it can be used up in nuclear burning," Howe said.