Stanford Report, November 6, 2001
Scientists solve centuries-old query: What's inside a sunspot?
BY MARK SHWARTZ
A new study is shedding light on one of the great mysteries of astrophysics -- the origin of sunspots.
By analyzing sound waves ricocheting inside the sun, a team of Stanford-based scientists has produced the first detailed image of the inner core of a sunspot. Their findings reveal fast-moving streams of hot plasma -- flows of electrically charged gas converging into a gigantic vortex that penetrates the solar surface.
Scientists have speculated about what lies beneath sunspots since the early 1600s, when Galileo first reported seeing mysterious, Earth-sized blotches on the face of the sun.
"Until now, we've looked down at the top of sunspots like we might look down at the leaves in treetops," says Thomas L. Duvall Jr., a visiting astrophysicist from the NASA Goddard Space Flight Center.
"For the first time we're able to observe the branches and trunk of the tree that gives it structure," he notes, "but the roots are still a mystery."
Duvall and Stanford collaborators Junwei Zhao and Alexander G. Kosovichev describe their discovery in the Aug. 10 issue of the Astrophysical Journal.
"The significance of our study is that we are the first to observe the actual dynamics of sunspots just below the visible surface," says Kosovichev, a senior research scientist at Stanford's W. W. Hansen Experimental Physics Laboratory (HEPL).
"What we found is that sunspots aren't static but consist of very strong, downward flows of plasma traveling toward the interior of the sun at speeds of about 3,000 miles per hour," he says.
Understanding the forces that give rise to these plasma flows will be useful in our daily lives, Kosovichev adds, because sunspots often are accompanied by powerful solar storms that can disrupt radio communications, power grids and orbiting satellites.
To chart the interior of a sunspot, the Stanford team used data obtained from the Solar and Heliospheric Observatory (SOHO) -- a research satellite positioned about a million miles from Earth.
Launched in 1995 and jointly operated by NASA and the European Space Agency, SOHO carries an electronic instrument called the Michelson Doppler Imager (MDI). The MDI maps the solar interior by measuring the velocity of sound waves passing through the sun. This technique, known as helioseismology, works on the same principle as medical ultrasound -- the process that allows doctors to "see" a fetus inside a pregnant woman.
"Solar sounds are naturally generated by solar convection when hot gases rise to the surface from the center of the sun," notes Zhao, a graduate student in physics and lead author of the Astrophysical Journal study. "It's similar to how ocean noise is produced by waves crashing at the surface."Data from the MDI/SOHO mission are routinely downloaded and analyzed at Stanford by a research team that includes Zhao, Kosovichev and Duvall.
"Before SOHO and high-resolution helioseismology, we could only study sunspots by observing the solar surface and above, but the real action is inside the sun," says Research Professor Philip H. Scherrer, principal investigator of the SOHO/MDI team at Stanford.
"With the MDI instrument aboard SOHO, we finally are able to use observations of sound waves that travel in the solar interior to map out the temperature and flow structure beneath spots," he explains.
Sunspots are regions of intense magnetic fields that can last several days or several weeks. The problem, says Scherrer, is that these magnetic fields all have the same polarity -- either positive or negative. So why do they stay clumped together instead of repelling each other and flying apart?
"Sunspots don't know they are supposed to quickly die," notes Scherrer. "Instead, they stubbornly remain alive for days or weeks at a time. This suggests some sort of confining motion is holding the magnetic fields together, but when we observe the surface of a spot, we only see outward flows of plasma."
Another mystery involves the solar cycle, during which sunspots gradually increase and decrease in number roughly every 11 years. The last solar minimum occurred in October 1996, when an average of only one spot appeared each day. The last maximum was in July 2000, when the mean daily sunspot number was 170. The sun is currently in a secondary peak of activity. The next minimum is expected in January 2007, and the next maximum in 2011.
"What we still do not know is why there is an 11- year solar cycle," notes Scherrer. "But thanks to this new study, we are finally learning how individual spots hold themselves together."
In their Astrophysical Journal study, the Stanford team used MDI satellite date to analyze a single sunspot that was visible on June 18, 1998. By measuring the speed of solar sound waves generated that day, the researchers were able to produce three-dimensional maps of a region extending some 10,000 miles below the sunspot.
"Sound-speed maps tell us about the distribution of temperatures and magnetic fields," notes Kosovichev. "That's because sound waves propagate faster in regions with higher plasma temperatures and stronger magnetic fields."
He points out that sunspots appear dark because they are about 3,000 degrees Fahrenhit cooler than their surroundings. As expected, analysis of the June 1998 sunspot revealed that sound waves travel about 10 percent slower at the surface where temperatures are lower, and maintain this relatively slow pace as they begin moving toward the interior of the sun. When the sound waves reach a point about 3,000 miles below the surface, however, their speed increases significantly, indicating that the roots of a sunspot are hotter than their surroundings.
"This means that sunspots are cool only to depths of about 3,000 miles -- a relatively shallow layer considering that it's about 430,000 miles from the surface to the center of the sun," Kosovichev explains.
The outflowing plasma, which scientists have long observed at the surface of sunspots, turns out to be just that -- a surface event. "If you can look a bit deeper," says Zhao, "you find material rushing inward, like a planet-sized vortex."
This inflow is strong enough to pull the magnetic fields together and reduce the amount of heat that normally flows from inside the sun. That explains why sunspots are cooler, and thus darker, than the surrounding surface.
"The cool sunspot continues to cool the material around it, which consequently sinks," observes Douglas Gough, professor of theoretical astrophysics at the University of Cambridge in England and co-investigator of the SOHO/MDI project.
"The converging plasma that is drawn in to replace the sinking material imposes an increased pressure on the sunspot, which prevents the magnetic field from dispersing and keeps the sunspot intact," he adds.
Using data from the study, NASA scientists put together a 3-D animation of a typical sunspot that shows a cluster of magnetic flux tubes held together by strong converging down-flows that plunge beneath the surface.
"In the deep interior, this magnetic cluster becomes a loose, spaghetti-like structure that allows plasma to flow through it," Kosovichev concludes."An analogy is my daughter's ponytail," adds Scherrer. "Without the rubber band to contain the strands of hair, they would fly in all directions. As it is, they are gathered only in the vicinity of the containment. If we could cut through just above the band, we would see a nice compact bundle. The flows we have found are like the rubber band."
"One of the striking features of these observations is just how shallow a sunspot is," notes Gough. "There have been purely theoretical debates in the past about how deep sunspots might be, but these observations have given us the answer."
Gough says the Stanford study may eventually provide clues to unraveling the secret of the sun's 11-year cycle and similar phenomena in other solar systems. "By understanding magnetic activity in other stars, we may in turn learn more about how the sun works, too," he concludes.
SOHO mission is officially scheduled to end in April 2003, although
NASA and European Space Agency administrators say that it may
continue several years beyond that date. NASA has announced plans
to replace SOHO in 2006 with a new satellite called the Solar
Dynamics Observatory (SDO), which for the first time will allow
researchers to monitor solar activity 24 hours a