One mystery that has long dogged solar physicists involves sunspots' habit of traveling in pairs. Each sunspot has just one polarity -- either positive or negative -- like one end of a battery. The magnetic energy from one sunspot loops outward into the solar atmosphere and reconnects to its pair, which travels behind it as they migrate across the Sun's surface.

But theories suggest that the single-polarity configuration should cause the magnetic filaments that make up a sunspot to simply fly apart, says Stanford researcher Phillip Scherrer. Imagine trying to hold together the ends of several magnets, each with the same polarity.

Yet the spots remain for days, sometimes weeks.

"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," Scherrer said.

The new view inside sunspots, provided by a spacecraft called the Solar and Heliospheric Observatory (SOHO), shows a previously unseen process that seems to resolve this puzzle. Plasma in the middle of a sunspot zooms toward the center of the Sun at 3,000 mph, creating a siphon of sorts that reigns in the magnetic fields.

Alexander Kosovichev, a member of the research team, explained what's going on:

Magnetic fields in sunspots are known to prevent the heat that's generated deep within the Sun from rising to the surface. So the plasma in a sunspot is cooler than plasma on the surrounding surface of the Sun. Since the sunspot plasma is cooler, it is heavier, and it plunges downward.

"That draws the surrounding plasma and magnetic field inward toward the sunspot's center," Kosovichev told SPACE.com. "The concentrated field promotes further cooling and sinking flow and draws in still more material. This sets up a self-maintaining cycle of material circulation."

The process is somewhat analogous to a hurricane.

A warm ocean heats air near the surface, which rises, pulling surface air inward from outside a hurricane. The inward rushing air forces more air to rise near the center of the storm, and a cycle is created that cannot be broken until the storm moves over cooler water or land.

In sunspots, the converging flow, said Kosovichev, "generates dynamic pressure, like in hurricanes, which holds the magnetic elements together."

"Until now, we've looked down at the top of sunspots like we might look down at the leaves in treetops," said Thomas L. Duvall Jr., an astrophysicist from NASA's Goddard Space Flight Center who also worked on the study. "For the first time we're able to observe the branches and trunk of the tree that gives it structure."

But the roots of sunspots are still a mystery, Duvall said.

And it's not clear whether or how the downward flow of plasma might trigger solar flares.

Flares usually occur when strong magnetic fields of two opposite polarities come close to each other and reconnect, Kosovichev said. The flows beneath the sunspots may have something to do with bringing these opposite polarities together, he said, but that is still being investigated.

Though sunspots typically appear in clusters, Kosovichev said the new study applies only to how individual sunspots are held together.

The SOHO satellite sits about a million miles from Earth and faces the Sun. It was launched in 1995 as a joint project of NASA and the European Space Agency.

The new study used data provided by a Doppler imager on SOHO. The instrument maps the Sun's interior by measuring the velocity of sound waves and watching how they bounce around. Known as "helioseismology," the technique works on the same principle as medical ultrasound.

"Solar sounds are naturally generated by solar convection when hot gases rise to the surface from the center of the Sun," said Junwei Zhao, a Stanford graduate student and lead author of this study, which appears in the Astrophysical Journal. "It's similar to how ocean noise is produced by waves crashing at the surface."

The study involved about a dozen sunspots but most of the data was collected on a single 1998 sunspot. NASA scientists have used the data to make a 3-D animation of a typical sunspot that shows a cluster of "magnetic flux tubes" held together by the downflows. About 3,000 miles down, the mechanism gives way and the tubes spread out.

The processes now seen by SOHO were suggested in 1974 by Friedrich Meyer and colleagues of Germanys Max-Planck-Institut. A similar expectation was put forth in 1979 by Eugene Parker.

"Our observation seems to provide strong evidence for both predictions," Zhao says.

The study might help to explain another curious solar phenomenon: When dark sunspots are most prevalent, the Sun actually appears brighter overall.

"The descending flow is readily able to extract the heat that accumulates beneath the spot," said Douglas Gough, a professor of theoretical astrophysics at the University of Cambridge in England and another member of the research team. "It then spreads the heat away from the sunspot and eventually brings it to the surface of the Sun far from the spot, from where it is radiated into space."

The study could also lead to clues that might one day explain why the Sun has an 11-year cycle of activity, Gough said.