This artist's conception shows a band of supergranules rotating faster across the Sun's surface than features like sunspots. Credit: NASA

The mystery of why large features called supergranules move across the Sun's surface faster than the Sun rotates has been solved, according to a team of scientists using the Solar and Heliospheric Observatory (SOHO) spacecraft. Instead of actually moving faster than the Sun, their apparent rapid rotation is an illusion generated by a pattern of activity, like fans doing the wave at a sporting event.

"This is fascinating because no theory or computer simulation had predicted what we observe," said Laurent Gizon of the W.W. Hansen Experimental Physics Laboratory (HEPL), Stanford University, Calif. Gizon is the lead author of a paper on this research that will appear in the January 2 issue of Nature.

Supergranules cover the Sun's visible surface (photosphere) in a network, called supergranulation, of cells: irregular bright regions that are horizontal outflows of electrified gas (plasma). Supergranule cells get their name from their resemblance to smaller features in the photosphere called granules. Granules are believed to be convection cells of plasma that transfer heat from the solar interior to the surface. They resemble the cells seen at the surface of a simmering pot of soup, although granules are much larger (about the size of Texas at 1,000 kilometers or about 620 miles across). Supergranules are larger still -- at around 30,000 kilometers (18,600 miles) across, they could comfortably frame two Earths. The whole solar surface is covered by several thousands of supergranules.

Data from SOHO's Michelson Doppler Imager (MDI) reveal that the pattern of supergranulation moves across the solar surface in waves. MDI analyzes velocity images of the solar surface to infer the movement and structure of plasma on the surface and deep in the interior. MDI data allowed the team to determine that the supergranulation propagates around the Sun like waves, explaining why it appears to rotate faster than expected.

"When people in a stadium do the wave, nobody actually moves in the direction of the wave -- they just jump up and sit down. In the same way, we discovered that individual supergranule cells don't really move faster than the solar surface. Supergranulation is just a pattern of activity that is moving across the solar surface in waves," said Dr. Tom Duvall, of NASA's Goddard Space Flight Center, Greenbelt, Md., stationed at HEPL.

Supergranules are visible in "dopplergrams." A dopplergram shows motions on the solar surface with areas moving toward us appearing dark, and away, bright. It can also be thought of as a map of the speed of the Sun's surface. Credit: NASA/ESA

With the mystery solved, the next step is to determine the mechanism for the pattern of activity that generates supergranulation waves. The mechanism is unknown, but a clue might be found in the nature of the waves. The waves actually travel in all directions across the solar surface, but for some reason, they are stronger (have a greater amplitude) in the direction of the solar rotation. It is this bias that gives rise to the illusion of movement faster than solar rotation in the first place, since the waves moving with the Sun's rotation are most prominent. The team is hopeful that this characteristic will help them discover how they work.

"It is quite difficult to speculate about the physical origin of the phenomenon. But it is likely that rotation itself is at the origin of the supergranulation waves," said Gizon.

Solar scientists are hopeful that these clues about supergranulation behavior will also clarify the mysterious nature of supergranules themselves. Supergranules remain mysterious because there is no explanation for their characteristic size of 30,000 kilometers. The depth of the supergranulation layer is also unknown.

The team used MDI data from 1996, a time when violent solar activity was less frequent, so the wave patterns could be seen clearly. The Sun moves through an 11-year cycle of activity, from quiet to stormy and back again, and the team will use data from more recent, stormier periods to see if the level of solar activity affects the waves somehow. A good understanding of supergranulation would help understand how magnetic fields are transported and dispersed near the solar surface. Understanding the dynamics of solar magnetism is important because scientists believe rapid changes in solar magnetic fields power violent solar activity, like flares and coronal mass ejections.