Radiative zone

Radiative zone

From about 0.25 to about 0.7 solar radii, solar material is hot and dense enough that thermal radiation is sufficient to transfer the intense heat of the core outward.[45] In this zone there is no thermal convection; while the material grows cooler as altitude increases (from 7,000,000 K to about 2,000,000 K) this temperature gradient is less than the value of adiabatic lapse rate and hence cannot drive convection.[34] Heat is transferred by radiation—ions of hydrogen and helium emit photons, which travel only a brief distance before being reabsorbed by other ions.[45] The density drops a hundredfold (from 20 g/cm3 to only 0.2 g/cm3) from the bottom to the top of the radiative zone.[45]

Between the radiative zone and the convection zone is a transition layer called the tachocline. This is a region where the sharp regime change between the uniform rotation of the radiative zone and the differential rotation of the convection zone results in a large shear—a condition where successive horizontal layers slide past one another.[46] The fluid motions found in the convection zone above, slowly disappear from the top of this layer to its bottom, matching the calm characteristics of the radiative zone on the bottom. Presently, it is hypothesized (see Solar dynamo), that a magnetic dynamo within this layer generates the Sun's magnetic field.[34]




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