First glimpse inside a sunspot

The data produce a 3D image of the sunspot

By BBC News Online science editor Dr David Whitehouse

By analysing sound waves travelling inside the Sun, a US team of scientists has produced the first detailed image of what goes on inside a sunspot.

We are the first to observe the actual dynamics of sunspots

Stanford team

The picture that emerges is of fast-moving streams of hot, electrically charged gas converging into a gigantic vortex that reaches below the solar surface.

Scientists have speculated about what lies beneath sunspots since the early 1600s, when astronomers first reported seeing mysterious, Earth-sized blotches on the face of our star.

Finally, we have some answers. "The significance of our study is that we are the first to observe the actual dynamics of sunspots just below the visible surface," said Alexander Kosovichev of Stanford`s Hansen Experimental Physics Laboratory.

Going inside

"What we found is that sunspots are not static but consist of very strong, downward flows of plasma - electrically charged gas - travelling toward the interior of the Sun at speeds of about 4,800 kilometres per hour (3,000 miles per hour)," Kosovichev said.

A sunspot is a shallow phenomenon

To map the interior of a sunspot, the Stanford team used data obtained from the Solar and Heliospheric Observatory (Soho) satellite positioned about 1.6 million km (a million miles) from Earth.

They used Soho's Michelson Doppler Imager (MDI) - a device that 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 foetus inside a pregnant woman.

"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," said Professor Philip Scherrer, principal investigator of the Soho/MDI team at Stanford.

"With the MDI instrument aboard Soho, we are able finally to use observations of sound waves that travel in the solar interior to map out the temperature and flow structure beneath spots," he added.

Sunspot study

The Stanford team analysed a single large sunspot that was visible on 18 June, 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 about 16,000 km (10,000 miles) below the sunspot.

The large sunspot that was visible on 18 June, 1998

Analysis of the giant sunspot revealed that sound waves travel about 10% 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 4,800 km (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 4,800 km (3,000 miles) - a relatively shallow layer considering that it is about 692,000 km (430,000 miles) from the surface to the centre of the sun," Kosovichev said.

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," said Stanford's Junwei Zhao, "you find material rushing inward, like a planet-sized vortex."

This inflow is strong enough to 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," said Douglas Gough, professor of theoretical astrophysics at the University of Cambridge in England.

"One of the striking features of these observations is just how shallow a sunspot is," noted 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 said the Stanford study might eventually provide clues to unravelling 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 concluded.