Abstract:
Observations suggest that the large-scale convective velocities obtained by solar convection simulations might be over-estimated (convective conundrum). One plausible solution to this could be the
small-scale dynamo which cannot be fully resolved by global simulations. The small-scale Lorentz
force suppresses the convective motions and also the turbulent mixing of entropy between upflows and
downflows, leading to a large effective Prandtl number (Pr). We explore this idea in three-dimensional
global rotating convection simulations at different thermal conductivity (κ), i.e., at different Pr. In
agreement with previous non-rotating simulations, the convective velocity is reduced with the increase
of Pr as long as the thermal conductive flux is negligible. A subadiabatic layer is formed near the
base of the convection zone due to continuous deposition of low entropy plumes in low-κ simulations.
The most interesting result of our low-κ 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 thus establish an anti-solar
differential rotation, in striking contrast to the solar rotation profile. If such low diffusive plumes,
driven by the radiative-surface cooling, are present in the Sun, then our results cast 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 the downward plumes must
take into account the angular momentum transport and heat transport. Published by AIP Publishing.
