High-resolution calculation of the solar global convection with the reduced speed of sound technique: II. Near surface shear layer with the rotation

TL;DR

This paper presents high-resolution simulations of solar convection near the surface, revealing the formation and maintenance of the near surface shear layer through complex turbulent and rotational interactions.

Contribution

It introduces a novel simulation approach extending to 0.99 solar radii, capturing small-scale convection and elucidating the mechanisms behind the near surface shear layer.

Findings

NSSL forms mainly at high latitudes in the upper 0.025 solar radii.

Reynolds stresses transport angular momentum inward, driving the NSSL.

Correlations of turbulent velocities vary with latitude and influence the shear layer.

Abstract

We present a high-resolution, highly stratified numerical simulation of rotating thermal convection in a spherical shell. Our aim is to study in detail the processes that can maintain a near surface shear layer (NSSL) as inferred from helioseismology. Using the reduced speed of sound technique we can extend our global convection simulation to $0.99\,R_{\odot}$ and include near the top of our domain small-scale convection with short time scales that is only weakly influenced by rotation. We find the formation of a NSSL preferentially in high latitudes in the depth range $r=0.95-0.975R_\odot$. The maintenance mechanisms are summarized as follows. Convection under weak influence of rotation leads to Reynolds stresses that transport angular momentum radially inward in all latitudes. This leads to the formation of a strong poleward directed meridional flow and a NSSL, which is balanced in the meridional plane by forces resulting from the $\langle v'_r v'_\theta\rangle$ correlation of turbulent velocities. The origin of the required correlations depends to some degree on latitude. In high latitudes a positive correlation $\langle v'_rv'_\theta\rangle$ is induced in the NSSL by the poleward meridional flow whose amplitude increases with the radius, while a negative correlation is generated by the Coriolis force in bulk of the convection zone. In low latitudes a positive correlation $\langle v'_rv'_\theta\rangle$ results from rotationally aligned convection cells ("banana cells"). The force caused by these Reynolds stresses is in balance with the Coriolis force in the NSSL.