Macroscopic transport -: Large-scale advection, turbulent diffusion, wave transport

While the solar convection zone is very well mixed by its turbulent motions, chemical composition gradients build up in the radiative interior due to microscopic diffusion and settling, and to nuclear burning. Standard models, which ignore any type of macroscopic transport, cannot explain the depletion of lithium in solar-type stars, as they evolve; neither do they account for the observed profile of molecular weight at the base of the solar convection zone. Such macroscopic transport can be achieved through thermally driven meridian currents, through turbulent diffusion generated by differential rotation and possibly through gravity waves. These processes transport also angular momentum, and therefore the internal rotation profile of the Sun provides a crucial test for their relative importance. So does also the behavior of tidally locked binaries, which appear to destroy less lithium than single stars of the same mass.