SOI-MDI Newsletter
SOI-MDI Newsletter
March 13-15 SOI/VIRGO Team Meeting, StanfordMarch 29-April 4 Cambridge Workshop
April 21-25 EGS `97, Vienna
June 10-12 5th SOHO Workshop, Oslo
August 18-30 IAU Meeting, Kyoto
September 15-19 SOI/GONG Meeting, Stanford
September 22-25 ESLAB Symposium, ESTEC
From the looping shape of fiery solar flares, to the varying intensity of the solar winds, to the mysterious 11-year sunspot cycle, large-scale events on the surface of the Sun are controlled by strong magnetic fields generated deep in its interior.
The location of the solar dynamo where these fields originate has been something of a mystery. But now a team of Stanford scientists has narrowed the search for this region to a layer 38,000 miles thick and centered at a depth of about 135,000 miles below the solar surface. In this region they have found evidence for two conditions indicative of the dynamo's presence: a high level of turbulence and shear flows caused by changes in rotation rate. ``Understanding the solar dynamo is extremely important to our overall understanding of the Sun's behavior. With these new data we can look forward to a rapid increase in our understanding of this key feature,'' said Philip H. Scherrer, the principal investigator of Stanford's Solar Oscillations Investigation team.
Scherrer reported the new, more precise location of the dynamo layer at the American Geophysical Union meeting in San Francisco. He provided the information to fellow scientists in a poster paper and to the media in a news conference on Tuesday, Dec. 17.
The ability to track the solar dynamo's location with increased precision and to measure the conditions there with greater accuracy should allow the researchers to determine how the region changes during the solar cycle. Currently, the Sun is at its least active. As it moves into a more active phase the scientists should be able to determine whether the dynamo layer changes in location or thickness, and if it grows hotter or colder, Scherrer said.
The Stanford instrument, a Michelson Doppler Imager, measures the vertical motion of the Sun's photosphere simultaneously at a million different points and can detect surface movements as slow as a millimeter per second. Much of the Sun's surface motion is due to stationary ``standing waves'' created by sound waves that ricochet around the Sun's interior. These waves have periods ranging from about three to 20 minutes, considerably below the range of human hearing. Because the density of solar gases increases with depth, the waves are continually being refracted as they travel inward, forcing them to follow curving paths that bring them back to the surface where they are reflected downward again. As they do this repeatedly, they produce ``standing waves'' that cause different areas of the Sun's surface to vibrate in and out of phase in discernible patterns.
Shorter waves do not travel deeply, but the longer waves penetrate nearly to the Sun's center. So scientists can use them to obtain information about conditions within the Sun that otherwise are hidden beneath its fiery exterior. This is called helioseismology.
Helioseismology can provide information about temperatures and elemental composition in the Sun's interior because changes in these factors affect the speed with which pressure waves travel. For a similar reason, the technique also provides estimates of the how the rotation rate varies with depth. Such an analysis was the key to locating the position of the solar dynamo. In the 1970s solar physicists placed it in the convection zone, the outer 30 percent of the Sun where its gases boil much like water in a pot on the stove, forming giant cells of rising and falling gases that carry the heat to the surface. But in the early 1980s they realized that features could not persist long enough in this area to act as a dynamo. So attention shifted to the region just below the convection zone.
Ground-based helioseismic data, however, indicated that the shear layer in this region was fairly broad and overlapped significantly with the convection zone where the dynamo could not exist.
``This produced a lot of skepticism about the dynamo theory itself,'' reports Alexander Kosovichev, a senior research scientist in SOI who did much of the current analysis. It even led some scientists to propose that the Sun's magnetic field might be locked in its core, he says.
Four months of SOHO observations have resolved this apparent problem. They show that the shear layer is sharper and does not extend into the convection zone as suggested by ground-based measurements. Also, they indicate that sound waves speed up more than expected in this region, indicating that the turbulence and mixing associated with a dynamo are present. At this distance, which is about two-thirds of the way from the Sun's center to its surface, the Sun undergoes a fundamental change in the nature of its rotation. Inside this boundary the Sun rotates like a solid object. Closer to the surface the rotation rates at different latitudes begin to diverge, with the equatorial regions rotating more rapidly than the middle latitudes and polar regions.
Figure 1: Sound speeds within the Sun relative to a standard solar model. The red color corresponds to a faster speed, and the blue color denotes a slower speed of sound. The latitudinal variation near the surface, probably, denote temperature differences, with hot (red) and cold (blue) regions. A sharp decrease in the sound speed, detected at the boundary of the energy-generating core, may be attributed to material mixing by unstable nuclear burning processes. An increase in sound speed is located in a thin layer just below the convection zone where a sharp variation in rotation rate is also detected, generating turbulence that may mix material near it.
Figure 2: Sketch of the toroidal magnetic flux system generated by the
solar dynamo below the bottom of the convection zone, and the active
regions formed by emerged magnetic loops (adopted from N.Brummell,
F. Cattaneo, J.Toomre, Science, v.269, p.1370, 1995).
Figure 3: The equatorial rotation rate (top) and the relative differences
between the squared sound speed in the Sun and a standard solar model
(bottom) as inferred from 2 months of the MDI data.
``The sun has mountains. These bumps are about five times the diameter of Earth,'' Jeffrey R. Kuhn, a solar physicist from Michigan State University and the National Solar Observatory in Sunspot, N.M., said Tuesday.
However, the mountains are only about a third of a mile high, he said.
Kuhn announced his results at the American Geophysical Union fall meeting, before colleagues had a chance to study them -- or before NASA, which funds some of the solar physics research, could announce it with any fanfare.
``Some of us have seen it for the first time today. I'm just very surprised at the result,'' said Ed Rhodes, a helioseismologist at the University of Southern California. ``I don't think this is at all impossible. I think it's potentially very exciting.''
The images were captured by the Michelson Doppler Imager, an instrument that can detect an object the size of a quarter at the edge of the moon from Earth.
It is one of a dozen instruments carried aboard the Solar and Heliospheric Observatory, a joint U.S.-European space mission launched on Dec. 2, 1995.
The imager detected about 60 bumps along the curve of the sun's profile; because it only can see the edges, there are likely hundreds of mountains of gases on the sun.
Kuhn said the gaps between the mountains are as wide as the mountains themselves -- or about five times Earth's diameter of 7,926 miles. The sun is nearly 1 million miles across.
Kuhn suspects the interaction of boiling gases and the sun's powerful magnetic field is forming the masses.
The $75 million imager, built by Lockheed Martin in Palo Alto, Calif., and developed by the Stanford Lockheed Institute for Space Research, makes highly sensitive measurements of sound waves every 12 minutes to detect the shape of the sun.
The instrument also showed that contrary to some theories, the sun is almost a perfect sphere. The instrument found that it's distorted by only about one mile of its nearly 1 million-mile diameter.
The imager, one of three U.S. instruments aboard the 2-ton spacecraft, began an expected two years of operation last May.
[1996 Associated Press]
Schoolchildren are taught the sun is shaped like a sphere. Later, in high school or college, they learn it's actually more like a flattened melon, with a love handle-like bulge around its middle.
But scientists have debated the exact size and shape of the sun for decades. At stake is a venerable idea, Einstein's theory of general relativity, the bedrock of modern cosmology.
Now, while trying to measure the sun's shape using a space satellite, scientists from Stanford University and other institutions have made an unexpected discovery. The sun is covered with enormous ``bumps,'' each one about 40,000 miles wide - five times the diameter of Earth - but very low, like flattened hills, roughly a few thousand feet high.
``It's extremely exciting. It's the kind of thing that, having seen, we definitely want to understand,'' space scientist Ed Rhodes of USC said Tuesday at the American Geophysical Union conference at Moscone Center.
Scientists have no explanation for the bumps. They may be like bubbles atop a boiling pot of water and, therefore, a hint of the convective forces churning away within the sun. Or they may reveal how the sun's tangled magnetic field molds its super hot surface of ionized gases and free-flying electrons.
The satellite ``is a very powerful telescope with which we can see marvelous things which nobody can understand. I fear here we are in heavy water where simple solutions won't suffice,'' said Werner Däppen of USC.
Unlike water-pot bubbles, the sun's bumps are remarkably stable, lasting at least a month each.
The discovery was made by a scientific instrument aboard the $1 billion solar and heliospheric observatory or SOHO, launched last year by NASA. Stanford scientists involved in the discovery were Phil Scherrer, Rock Bush, Rick Bogart and Luiz Sa. The instrument - the $75 million Michelson doppler imager, or MDI - was built by scientists from Stanford and Lockheed in Sunnyvale. The MDI is so sensitive that it can detect a change in the sun's shape as slight as 10 feet.
``That's as precise as measuring something as small as a quarter on the moon,'' said Jeffrey Kuhn of the National Solar Observatory in Sunspot, N.M.
MDI is doubly aptly named. Doppler refers to the technique by which it measures solar shape - by gauging subtle Doppler shifts in the frequency of sunlight as the Sun quakes like a huge bowl of Jell-O.
And Michelson refers to Albert Michelson, a 19th century U.S. physicist who, with Edward Morley, discovered that the speed of light never changes. Their finding paved the path to Einstein's theory of general relativity, a new theory of gravity.
The Einstein theory's most celebrated application occurred in 1919. Scientists used it to correctly forecast that during a total eclipse, the Sun's gravitational field would shift the apparent position of a nearby star.
Despite that success, skeptics pointed out the forecast was based on the assumption the sun was a slightly flattened - oblate - sphere. What, they asked, if it's really more or less oblate than assumed? That might invalidate Einstein's forecast.
For this and other reasons, astronomers spent a few decades trying to precisely measure the sun's shape. In the 1970s, some experts said the sun is tubbier than Einstein assumed - 10 miles wider along its equator than measured from pole to pole.
Now SOHO has come to Einstein's rescue by observing the sun from space, unhampered by Earth's turbulent atmosphere. SOHO shows the sun is fairly slim, only ``a mile shorter than it is fat,'' Kuhn said. This ``tells us general relativity as Einstein conceived it is safe.''
[The San Francisco Examiner]
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