Colloquium
Why supergranulation? The structure of the upper superadiabatic transition in the Sun and stars

Abstract

Turbulent convection in stellar envelopes is critical to heat transport and dynamo activity. Modeling it well has proven surprisingly difficult, and recent solar and stellar observations have raised questions about our understanding of the dynamics of both the deep solar convection and the mean structure of the upper layers of convective stellar envelopes.  In particular, the amplitude of low wavenumber convective motions in both local area radiative magnetohydrodynamic and global spherical shell magnetohydrodynamic simulations of the Sun appear to be too high. In global simulations this results in weaker than needed rotational constraint and consequent non solar-like differential rotation profiles. In deep local area simulations it yields strong horizontal flows in the photosphere on scales much larger than the observed supergranulation, leaving the origin of the solar supergranular scale enigmatic. The problem is not confined to the Sun. When comparing computed oscillation frequencies to observations, mixing length models of stellar convection show too sharp a transition to the interior adiabatic gradient. This contributes to what asteroseismologists call the `surface effect' correction.

 We suggest that there is a common solution to these problems: convective motions in stellar envelopes are even more nonlocal than numerical models suggest. Small scale photospherically driven motions dominate convective transport even at depth, descending through a very nearly adiabatic or possible even somewhat subadiabatic deep convection zone. Convection of this form may meet Rossby number constraints set by global scale motions, and implies that the solar supergranulation is the largest buoyantly driven scale of motion in the Sun. We test this hypothesis using a suite of three-dimensional stellar atmosphere models, and can use it to both recover their mean stratification and estimate the supergranular scale on other stars