Observational constraints on solar convection at large and intermediate scales

Observational methods have provided a wealth of information about solar convection, which takes place in the low Prandtl number regime. Both correlation tracking and helioseismology have been used to measure convective flows at large and intermediate scales. Correlation tracking methods use time series of images to follow the motion of features at the solar surface and produce measurements of the horizontal flows at the surface of the Sun. Helioseismology, the study of solar acoustic and surface-gravity waves, is used to infer the convective flows in the near-surface layers of the Sun. Measurements of large-scale flows from both of these methods suggest that the large-scale convection is weaker than predicted by simulations. At intermediate (supergranulation) scales, the near-surface convection on the Sun evolves in a wave-like pattern in which, on average, sites of horizontal divergence are replaced by sites of horizontal convergence after roughly two days. There is not yet a generally accepted explanation for this pattern.