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COMMENT: Prof. Phillip H. Scherrer, Physics (415) 723-1504

Stanford space experiment designed to take Sun's pulse

STANFORD -- Despite its closeness and vital importance to Earth, the Sun has proven effective at keeping its secrets:

How does its power source work? What causes sunspots, giant flares and other forms of solar activity? How long will the Sun continue to provide the energy required to sustain terrestrial life?

Efforts to answer these basic questions have led to major advances in physics and astrophysics, but the fundamental questions remain unanswered.

Now a new generation of instruments and techniques is poised to pierce the Sun's fiery veil, seeking information about conditions far beneath the photosphere, the Sun's visible surface. Several of these instruments, including one from Stanford University, are mounted on the European Space Agency's Solar and Heliospheric Observatory (SOHO), a $1.1 billion spacecraft scheduled for launch at the Kennedy Space Center in Florida on Thanksgiving Day, Nov. 23.

The Stanford experiment and two others carried on the spacecraft are designed to study the solar interior, using a technique called helioseismology. About 30 years ago, scientists realized that the Sun vibrates like a giant bowl of gelatin, and that information about the size and frequency of its pressure waves can provide important data about the Sun's interior. Because this type of research was analogous to methods that geophysicists employ to study the center of earth using sound waves from earthquakes, the field was named helioseismology.

The SOHO spacecraft also supports nine other experiments aimed at shedding new light on the corona and the solar wind. After SOHO's launch, it will be propelled to a point 1 percent of the distance from the Earth to the Sun: about 930,000 miles from Earth. From this vantage point, its instruments will study solar features for at least two, and possibly as many as six, years.

Stanford's experiment, the Solar Oscillations Investigation, is a $75-million joint project between Stanford and the Lockheed Palo Alto Research Laboratory. It uses an instrument called a Michelson Doppler Imager to measure the vertical motion of the Sun's surface in unprecedented detail, imaging the Sun with a one-million-element camera. The light is filtered so that it measures the shifts in color caused by the motion of the photosphere. With this technique, the instrument can detect surface movements as slow as a millimeter per second.

"Our instrument will generate two-thirds of the data from SOHO," said Phillip H. Scherrer, principal investigator and professor of physics at Stanford.

All these data on the movement of the Sun's surface are designed to map its many oscillations, Scherrer said. "The sun 'rings' simultaneously in thousands of different modes. An analogy is a bell in a sand storm," he said. This ringing takes place at frequencies far too low for the human ear to hear. These vibrations come in a variety of different modes, or patterns of motion. For example, the entire sun vibrates in and out. At the same time, four quadrants of the sphere oscillate together. In another mode, 10 areas, like the divisions on a soccer ball, are linked.

These motions are surface manifestations of waves that can penetrate deeply into the sun's interior, and their frequency and amplitude provide information about temperature, pressure and density inside the sun. The lower the mode - the smaller the number of vibrating regions - the deeper the waves penetrate.

According to current understanding, the sun's interior is divided into three very different regions:

  • The core, which extends out about 100,000 miles from the Sun's center, contains about half of its mass and generates about 98 percent of its energy through processes of nuclear fusion. Temperatures there are thought to be about 15.8 million degrees Celsius.
  • A relatively stable "radiative zone" extends for about 200,000 miles from the core to about 125,000 miles from the surface. In this region energy spreads outward by a process similar to the way in which a radiator distributes heat in a cold room. Temperatures gradually drop from 15.8 to 2 million degrees.
  • At about 125,000 miles below the surface, radiative transfer switches to convection, and the Sun's gases boil much like water in a pot on a stove, forming giant cells of rising and falling gases that carry the heat to the surface.

A primary goal of the Stanford experiment is to learn more about the convection zone.

"If we can measure the convection in [that] zone, then we have a good chance of understanding the processes that produce sunspots and active regions," Scherrer said. This also should provide information about the forces that give rise to the 11-year solar cycle, which affects both sunspots and solar flares.

The scientists also hope to shed light on the source of the magnetic fields that control much of the Sun's surface activity. Because the hydrogen and helium gases that make up most of the Sun's mass are electrically charged, their movements are affected by the Sun's powerful magnetic fields. That is why solar flares invariably take the form of large loops. According to current thinking, the flowing, electrically charged fluid that produces these fields is located at the base of the convection layer.

In addition to studying the pressure waves, scientists also hope to measure the surface effects of gravity waves. These are interior waves that travel only beneath the convection zone. They never have been detected, but scientists calculate that they should produce subtle surface features, resulting in motions equivalent to hundreds of yards per hour.

If such waves can be measured, they are expected to provide valuable information about the temperature and density in the core, where the nuclear reactions take place. If these reactions work the way scientists believe, they should be producing larger numbers of neutrinos - ghostly particles with virtually no mass - than scientists have been able to detect.

To match the current measurements of solar neutrinos, using standard nuclear physics, requires significantly lower core temperatures and densities than earth-based helioseismic measurements and current solar models predict. But if SOHO confirms the higher numbers, "then we will be forced to go to strange physics or give neutrinos some mass" to explain the discrepancy, Scherrer said.

The 30-member Stanford group includes faculty members, research scientists, computer programmers, students and staff. In addition to Scherrer, Stanford faculty involved are Arthur B.C. Walker Jr., professor of physics; Peter A. Sturrock, professor of applied physics; and Ronald N. Bracewell, professor emeritus of electrical engineering. Research scientists participating include Rock Bush, Rick Bogart, J. Todd Hoeksema and Jesper Schou.

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An image of the sun taken with the Stanford Michelson Doppler Imager that is flying on the European Space Agency's Solar and Heliospheric Observatory (SOHO) due to launch on Nov. 23. The image shows the movement of the Sun's photosphere. Light and dark areas show outward and inward motion.

-dfs-

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