Turbulent Convection

In the central third of the Sun, hydrogen gas converts to helium, creating a temperature of 16 million degrees C. That heat radiates outward, but the radiation slows about two-thirds of the way to the Sun's surface. As a result, the intensely heated gases in the outer portion of this mid-zone expand, becoming less dense, and rise by buoyancy toward the surface. At the same time, cooler surface gases contract and sink back toward the mid-zone. This convective overturning process is very rapid, in order to carry the large amount of heat, and turbulence ensues.

In this convection zone, roughly the Sun's outer third, different regions rotate at different rates. A spot painted on the equator, for instance, would go around in about 25 days, but near the poles a similar spot would take 33 days to fully rotate. "We're keenly interested in this," Toomre says, "because we think it ties in very closely with how stars like the Sun build and rebuild their magnetic fields. For instance, we know the Sun has 11-year cycles during which the polarity of the north and south poles changes. Is the building and rebuilding of magnetic fields taking place in the highly turbulent region? These are some of the big physics questions of the past few decades, and we need to understand them if we are going to have some comfort in predicting the Sun's cycles and variability."

While the Earth has gone through many ice ages, notes Toomre, their causes aren't well understood. "Is it because the planetary orbits have been jiggled, or is it partly because the heat flow from the Sun has changed? It also could be caused by particle flows from the Sun. Then the issues of rotation stability and magnetic action become important."

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