Giant Convective Cells Found On Sun After 30-Year Search

May 28, 1998

Things the size of Jupiter should be pretty hard to hide, especially when they're staring at us from the face of the Sun.

Still, it's taken almost 30 years of hunting to find giant convection cells that may play a major role in how the Sun rotates and how sunspots move across its face and even influence space weather.

"This is kind of like finding the high- and low-pressure systems that govern the weather on the Earth," said Dr. David Hathaway, a solar physicist at NASA's Marshall Space Flight Center. He presents his findings today to the American Geophysical Union's annual spring meeting in Boston. Colleagues working with him are Dr. R.S. Bogart and Dr. J.G. Beck, both of Stanford University.

That giant convection cells have only now been discovered - after almost 400 years of observing the sun by telescope - is because their movements are buried in the more violent, small-scale activities on the Sun. Not until the last few years have modern computers and advanced telescopes made it possible to see the cells.

Blue light specials

The motion of the Sun's atmosphere is measured by both red and blue shifts. Like the pitch of a siren rising as it approaches then falling as it recedes, the color of light is shifted toward the blue end of the spectrum if an object is moving towards you, and to the red end if it is moving away. The red shift is most familiar as a yardstick for measuring the size and age of the universe.

In studying the Sun, both blue and red shifts are quite important because gases are rising and falling and spreading out across the solar surface. The Michelson Doppler Interferometer aboard the Solar and Heliospheric Observatory (SOHO) measures the red shifts in light emitted by nickel, a trace gas on the sun.

"The biggest signal is solar rotation," Hathaway explained as he displayed a Doppler shift image of the Sun showing blue on the side rotating towards the observer and red rotating away. A thin white line runs down the middle because rotational motion there is sideways with respect to the observer.

The Michelson Doppler Interferometer was designed for helioseismology, the relatively new field that treats the sun as a ringing object, like a pressurized cola bottle ringing when tapped. With the Michelson Doppler Interferometer , scientists have uncovered millions of distinct vibrational patterns that are revealing details about the Sun's inner structure.

"I'm using it for something it wasn't strictly designed for, but is fully capable of doing," Hathaway explained. He was hunting giant cells which Bumba predicted must be at work on the sun.

The first step towards discovery was in the early 1960s when supergranules were found by Bob Leighton, George Simon, and Bob Noyes at the California Institute of Technology. In 1968 Simon along with Nigel Weiss (then at the Max Planck Institute) predicted that supergranules should be enclosed by larger convective cells that extend 150,000 km down to the bottom of the sun's convective zone.

Bigger also means smaller

The challenge is that as structures become larger, their motions are much smaller. Again using the weather analogy, tornadoes and hurricanes are energetic, but are driven by the larger flow patterns in the Earth's atmosphere.

"The flow of solar gases is more powerful than the magnetic fields," Hathaway said. "On the surface and below, they are in charge and carry magnetic ropes with them." Where the ropes erupt from the surface and loop into space and back is where sunspots appear.

But seeing through the weather to see the climate patterns was difficult. Nearly a hundred thousand images from the Michelson Doppler Interferometer were obtained at the rate of one a minute during June and July of 1996.

"First, we have to get rid of those 5-minute oscillations by averaging an hour's worth of data," he explained. "At that point, the data are ready for what I do."

What he does is measure all the different flow components in search of flows moving as slow a 1 m/s (2 mph) in the presence of material moving as fast as 2,000 meters per second in both directions. He also has to allow for the sun's rotation and the spacecraft's own motion.

Hathaway also applied spatial filters, special equations that draw out patterns that otherwise escape detection.

"What I do is break this image of the sun into its basic building blocks and retain only the biggest blocks," he explained. "Then I find the average velocity of different sized patches."

Finally, he built computerized movies so he could watch the patterns as the Sun rotated.

Giant weather patterns

"You can see, in the middle latitudes, things getting sheared off and relinking, reds with reds and blues with blues," he said, pointing to the computer screen. The blobs move around the solar disk, like a time-lapse movie of a satellite watching clouds over the Earth. And like a satellite image, patterns that set on one side of the Sun rise 14 days later on the opposite limb.

"This was the indication to me, as a scientist, that I was seeing a real pattern on the Sun," Hathaway explained.

The giant cells are indeed giant, large enough to swallow the planet Jupiter.

"They're slow - only 5 meters per second above the noise level - but they last a long time, and they're big in area," he said. Larger cells may be found, Hathaway said, but that would start to approach the size of the Sun itself.

Giant cells are far more than an oddity.

"I'm convinced that it should enhance our ability to predict space weather," Hathaway said, "just by looking at the Sun and seeing which way the winds blows." It will be a tad more complex than that. Studying the giant cells will help scientists predict where sunspots will go next. Sunspots are a key feature of active regions that pump X-rays, high-energy protons, and electrified gases into interplanetary space. In turn, these can directly affect satellites and power and communications systems on Earth.

Confirmation of giant cells and eventual understanding of their activities will also help us understand why the sun rotates as it does, faster at the equator and slower towards the poles.

"Differential rotation should be destroyed," Hathaway said. "Gas moving away from the equator should make polar gases move faster." The reason may be giant convection cells.

Hathaway noted that in the Geophysical Fluid Flow Cell, a "star in a test tube" model flown on two Space Shuttle missions, scientists were looking for "banana cells" that would solve the rotation dilemma.

Instead, they saw something more akin to the giant convective cells Hathaway has discovered. Continued observations with the Michelson Doppler Interferometer on SOHO may help determine whether the cells are carrying angular momentum away from the poles, as well as help determine the origins of the Sun's magnetic fields.

"This could open a new avenue for understanding the Sun," Hathaway said, "and for space weather and predicting magnetic fields."

NASA/Marshall Space Flight Center--Space Sciences Laboratory

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