The Role of Downflows in Establishing Solar Near-Surface Shear

TL;DR

This study uses numerical simulations to investigate how downflows and flow structures contribute to the formation of the Sun's near-surface shear layer, emphasizing the importance of specific flow patterns and meridional circulation.

Contribution

It demonstrates that a solar-like near-surface shear layer requires the interplay of unconstrained downflow plumes and Busse columns, highlighting the role of meridional circulation in shear formation.

Findings

High density contrast models produce near-surface shear confined to low latitudes.

Downflow plumes are more effective at high latitudes in transporting angular momentum inward.

Busse columns are essential for creating near-surface shear in the models.

Abstract

The dynamical origins of the Sun's tachocline and near-surface shear layer (NSSL) are still not well understood. We have attempted to self-consistently reproduce a NSSL in numerical simulations of a solar-like convection zone by increasing the density contrast across rotating, 3D spherical shells. We explore the hypothesis that high density contrast leads to near-surface shear by creating a rotationally unconstrained layer of fast flows near the outer surface. Although our high-contrast models do have near-surface shear, it is confined primarily to low latitudes (between $\pm15^\circ$). Two distinct types of flow structures maintain the shear dynamically: rotationally $\textit{constrained}$ Busse columns aligned with the rotation axis and fast, rotationally $\textit{unconstrained}$ downflow plumes that deplete angular momentum from the outer fluid layers. The plumes form at all latitudes, and in fact are more efficient at transporting angular momentum inward at high latitudes. The presence of Busse columns at low latitudes thus appears essential to creating near-surface shear in our models. We conclude that a solar-like NSSL is unobtainable from a rotationally unconstrained outer fluid layer alone. In numerical models, the shear is eliminated through the advection of angular momentum by the meridional circulation. Therefore, a detailed understanding how the solar meridional circulation is dynamically achieved will be necessary to elucidate the origin of the Sun's NSSL.