inside_sun_000330 If the sun can be thought to have a heart, then the currents of hot gas inside represent blood, pulsating around and upward through various layers toward the surface. Sunspots are the hemorrhaged artery in this analogy, bursting quagmires of pent-up energy.
Using sound waves to peer a third of the way into this energetic orb, researchers have found the pulse: Vast pools of hot, electrified gas rub shoulders as they move at different speeds, alternating between slow and fast paces in a surprisingly quick cycle thought to drive sunspot activity.
"For the very first time we have a handle on important variations deep inside the sun that are probably linked to the sunspot cycle," said a co-author of a new study, Jørgen Christensen-Dalsgaard of Aarhus University, Denmark. "Until now, our attempts to explain the complicated magnetic changes seen at the surface were pure speculation."
Sloppy rotation
At the equator, the sun rotates on its axis once every 27 days, roughly. But because it is not a solid object, other regions move at different speeds. It takes 35 days, for example, to make a full rotation near the poles.
Rotation rates also vary inside the sun, explains Rachel Howe, lead researcher on the study, which appears in the March 31 issue of the journal Science.
The outer third of the sun moves energy toward the surface by convection, a method of heat transfer that relies on the fact that heat rises. Below this convective zone is a region where energy moves outward via radiation, the speed-of-light transfer of energy via infrared waves.
Where the convective and radiative zones meet is a border region known as the tachocline. Researchers have suspected that sharp differences in rotation speed at this area would create a "dynamo" that generates magnetic fields. At times of high activity, this dynamo forces surrounding gas to create a surface eruption, or
sunspot.The new findings show that just above and below the tachocline, and close to the sun's equatorial plane, the rotation rate speeds up and slows down rhythmically every 16 months or so, Howe told SPACE.com. "When the material above the tachocline is moving faster than average, the material below is moving more slowly, and vice versa."
Each sphere shows bands of differing rotation at the tachocline (right half of each sphere). The white arc represents the tachocline. Blue streaks show slower rotation below the tachocline in the left image, a scenario reversed six months later in the right image. The left half of each sphere shows rotation rates on the sun's surface.
The region just above the border moves at about 45,900 feet (1,400 meters) per second, rotating once every 26 days, said Howe, who works at the National Solar Observatory in Tucson, Arizona. Just below the border, gas moves at 40,000 feet (1,200 meters) per second, requiring about 28 days to make a rotation.
How do you see inside the sun?
The discovery was made using the
SOHO spacecraft, which maintains a fixed perch partway between us and the sun, and the results were confirmed by ground-based telescopes of the Global Oscillation Network Group.Researchers analyzed sound for surface changes caused by waves reverberating through the sun's interior, a technique called helioseismology. The same principle uses earthquakes to study the structure of inner Earth.
Sunspot cycles and solar max
This 16-month cycle is much shorter than the roughly 11-year stretch between solar peaks, a more common solar cycle. The sun is currently near the peak of this cycle. During this so-called solar maximum, sunspot activity increases and more solar energy is sent earthward, potentially threatening satellites and power grids.
"We don't yet understand exactly how the cycles are related to sunspot activity, but because they are taking place in the tachocline, which is believed to be where the sunspot cycle originates, there must be a connection somewhere," Howe said. "It's one more piece of a puzzle that theorists may one day be able to fit together to explain the solar cycle."
The study also raised the question of whether there is a link between these observed deep changes and bands of gas nearer the surface. The surface "rivers" of gas that move parallel to the equator move slightly faster or slower than the average speed of the surrounding gas.
Near the sun's surface, bands of gas rotating at different rates are seen moving toward the equator over time.
Although the effect is subtle, it is persistent. Bands of fast and slow gas gradually move from high latitudes towards the equator over the years. Simultaneously, sunspots are sighted closer and closer to the equator as their 11-year cycle approaches its maximum of activity.
A glimpse into solar chemistry
The findings might lead to a better understanding of the chemistry of the convective zone, but the researchers caution that this part of the work is speculative.
The pulsing rotation rates might mean that material in the sun's relatively distinct layers is being mixed. There is less lithium at the sun's surface, for example, than researchers would expect.
"That might be explained if the lithium in the convection zone is being mixed down into the deeper layers, where it can be used up in nuclear burning," Howe said.
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