Supergranules move across solar surface by doing 'The Wave,' scientists conclude

BY BILL STEIGERWALD

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," says Laurent Gizon, a Stanford graduate student studying helioseismology in the W. W. Hansen Experimental Physics Laboratory (HEPL). Gizon is the lead author of a paper on this research that appeared in the Jan. 2 issue of Nature.

Supergranules cover the sun's visible surface (photosphere) in a network of "cells" called supergranulation. The cells are amorphous regions of horizontal outflows of electrified gas (plasma). Supergranules get their name from their resemblance to smaller features in the photosphere called granules. Granules are 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 width of Texas at 1,000 kilometers, or about 600 miles). Supergranules are larger still -- at around 30,000 kilometers (about 20,000 miles) across, they could comfortably frame two Earths. The whole solar surface is covered by several thousand supergranules.

Data from SOHO's Michelson Doppler Imager (MDI), for which Stanford physics Professor Philip Scherrer is principal investigator, reveal that supergranulation moves across the solar surface in waves. MDI images 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 supergranulation propagates around the sun like waves.

"When people in a stadium do the wave, nobody actually moves in the direction of the wave -- they just jump up and sit down," explains Tom Duvall, a scientist from NASA's Goddard Space Flight Center in Greenbelt, Md., who is stationed at HEPL. "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."

With the mystery solved, the next step is to determine the mechanism behind supergranulation waves. The mechanism is still unknown, but a clue might be found in an interesting property of the waves. As co-author Jesper Schou, a senior research scientist at Stanford, explains, "While the waves travel in all directions, they are stronger in the direction of rotation. This explains previous measurements which showed abnormally fast rotation. This is similar to looking at the ocean from a beach. Most of the waves are moving toward you, but the ocean is not moving toward you." The team is hopeful that this and other characteristics of the waves will help them discover the origin of the waves.

While supergranulation was first observed more than 40 years ago, the wavelike properties have only just been discovered. Says Gizon: "It is quite difficult to speculate about the physical origin of the phenomenon. But it is likely that the interaction between convection and rotation is at the origin of the supergranular waves."

Solar scientists are hopeful that these clues about supergranulation behavior also will elucidate the mysterious nature of supergranules themselves. 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, at 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. "An important property of the supergranulation is that it transports the magnetic fields near the solar surface, which may be a key part of the solar cycle," Schou says. Understanding the dynamics of solar magnetism is important because scientists believe rapid changes in solar magnetic fields power violent solar activity, such as flares and coronal mass ejections.

NASA funded the research presented in the Nature paper. SOHO is a mission of international collaboration between the European Space Agency and NASA.

Bill Steigerwald writes for NASA Goddard Space Flight Center. Stanford Report science writer Dawn Levy contributed to this report.