Supergranulation as the largest buoyantly driven convective scale of the Sun

TL;DR

This paper investigates the origin of solar supergranulation, proposing it as the largest buoyantly driven convective scale resulting from strong photospheric driving and non-local heat transport, supported by 3D simulations.

Contribution

It introduces a new interpretation of supergranulation as the largest buoyant convective mode driven by surface effects, explaining observed flow power spectra.

Findings

Supergranulation scale corresponds to the transition from buoyant driving to adiabatic stratification.

Simulations show the largest convective scales reflect the depth of buoyant driving.

Observed decrease in flow power at larger scales is due to rapid stratification transition.

Abstract

Supergranulation is characterized by horizontally divergent flows with typical length scales of 32 Mm in the solar photosphere. Unlike granulation, the size of which is comparable to both the thickness of the radiative boundary layer and local scale height of the plasma in the photosphere, supergranulation does not reflect any obvious length scale of the solar convection zone. Early suggestions that the depth of second helium ionization is important are not supported by numerical simulations. Thus the origin of the solar supergranulation remains largely a mystery. Moreover, observations of flows in the photosphere using either Doppler imaging or correlation or feature tracking show a monotonic decrease in power at scales larger than supergranulation. Both local area and global spherical shell simulations of solar convection by contrast show the opposite, a power law increase in horizontal flow amplitudes to low wavenumber. Here we examine this disparity, and investigate how the solar supergranulation may arise as a consequence of strong photospheric driving and non-local heat transport by cool diving plumes. Using three dimensional anelastic simulations with surface driving, we show that the kinetic energy of largest convective scales in the upper layers of a stratified domain reflects the depth of transition from strong buoyant driving to adiabatic stratification below. We interpret the observed monotonic decrease in solar convective power at scales larger than supergranulation to be a consequence of this rapid transition, and show how the supergranular scale can be understood as the largest buoyantly driven mode of convection in the Sun.