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 deplet ion 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 conv ection zone. Such macroscopic transport can be achieved through thermally driven meridia n 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 t idally locked binaries, which appear to destroy less lithium than single st ars of the same mass.