Colloquium: Unusual dynamics of convection in the Sun

The Sun is our nearest star; it is also the most important star that determines life on Earth. A large variety of phenomena observed on the Sun's surface, with potential impact on Earth, is thought to arise from turbulent convection in Sun's interior, this being the dominant mode of heat transport within the outer envelope at r ≳0.715 R_⊙. However, convection in the Sun differs in most of its aspects from convection processes known on Earth, certainly those under controlled laboratory conditions, thus seriously challenging existing physical models of convective turbulence and boundary conditions in the Sun. Solar convection is a multiscale-multiphysics phenomenon including the transport of mass, momentum, and heat in the presence of rotation, dynamo action, radiation fluxes, and partial changes in chemical composition. Standard variables of state such as pressure, mass density, and temperature vary over several orders of magnitude within the convection region, thus introducing immense stratification. Although the Sun has been explored intensely, observational evidence on the structure and intensity of turbulent convection processes remains indirect and essentially limited to observations of the granular convection patterns at the surface and helioseismologic data that probe the propagation of sound waves in the interior. In this Colloquium characteristic scales and dimensionless parameters are discussed, particularly from the perspective of laboratory convection, a research field that has progressed significantly in the last few decades. The estimates and calculations of solar conditions given here are based mostly on the standard solar model S of Christensen-Dalsgaard et al., which is a mean field model of solar convection. Light is shed on existing results to gain a deeper understanding of dynamical aspects of solar convection.