Unraveling Convection

In recent years, researchers have gleaned their understanding of the Sun's structure by measuring the frequencies of sound waves as they bounce around the Sun's inner layers and eventually leak out. Exactly how energy transfer occurs in the turbulent convection zone, however, is poorly understood. Direct observations show only the smooth, cellular surface of the Sun -- commonly known as solar granulation. Nevertheless, because of the types of gases and energy produced in the Sun, scientists believe that turbulent flows should be present.

Toomre and his colleagues have built a three-dimensional computer model of a small section of the convection zone. "We compute results for a box of compressible gas heated from below in such a manner that it is forced to convect rapidly," says Nic Brummell, a researcher in Toomre's group. In work to date, the group has shown that compressibility creates a smooth, laminar surface that disguises a turbulent interior when the model is forced. The computer modeling, in effect, makes it possible to see what's happening beneath the surface. "We now have access to sufficient resolution," says Brummell, "to study convective turbulence as opposed to laminar or mildly turbulent flows."

The simulations show that turbulence plays the major role in overall energy transport, reconciling observations and theory about transport mechanisms within the Sun's convection zone. Furthermore, the simulations show how turbulence may work in conjunction with strong rotation to produce the observed differential rotation profiles in the convection zone. Prior numerical models that didn't include turbulence had been unable to account for this.

"The resources at Pittsburgh," says Brummell, "allowed us to advance the modeling toward parameters more suitable for the solar case, and the underlying turbulence has now become apparent in the simulations."

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