http://arxiv.org/abs/1801.00560
Observational findings suggest that the convective velocity at large scales in global convection simulations might be over-estimated. One plausible solution to this could be that the solar convection is driven by the low entropy plumes (entropy rain). Another solution could be the magnetic field. The Lorentz force of the magnetic field suppresses the convective motions and also the turbulent mixing of entropy between upflows and downflows, leading to an increased effective Prandtl number ($\Pr$). We explore this idea in three-dimensional global rotating convection simulations at different thermal conductivity ($\kappa$), i.e., at different $\Pr$. In agreement with previous non-rotating simulations, the convective velocity is systematically reduced with the increase of $\Pr$. However, in rotating simulations, the convective velocity initially increases with the increase of $\Pr$ and when the thermal conductive flux becomes negligible, the convective velocity decreases. A subadiabatic layer is formed near the base of the convection zone due to continuous deposition of low entropy plumes in low-$\kappa$ simulations. The most interesting result of our low-$\kappa$ simulations is that the convective motions are accompanied by a change in the convection structure that is increasingly influenced by small-scale plumes. These plumes tend to transport angular momentum radially inward and establish an anti-solar differential rotation, in striking contrast to the solar rotation profile. If such low diffusive plumes, driven by the surface cooling, are present in the Sun, then our results reveal a serious doubt on the idea that a high effective $\Pr$ may be a viable solution to the solar convective conundrum. Our study also emphasizes that any resolution of the conundrum that relies on downward plumes must take into account angular momentum transport as well as heat transport.
B. Karak, M. Miesch and Y. Bekki
Wed, 3 Jan 2018
30/59
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