On the Amplitude of Convective Velocities in the Deep Solar Interior

TL;DR

This paper establishes lower bounds on deep solar convective velocities and length scales based on observed solar flows and fundamental MHD principles, challenging some existing dynamo models.

Contribution

It introduces a method using gyroscopic pumping and observed flows to constrain convective velocities and scales in the solar interior.

Findings

Convective velocities are at least 30 m/s near the surface and 8 m/s deeper down.

Deep convective motions have length scales no smaller than 5.5-30 Mm.

Results challenge the assumptions of certain flux-transport dynamo models.

Abstract

We obtain lower limits on the amplitude of convective velocities in the deep solar convection zone based only on the observed properties of the differential rotation and meridional circulation together with simple and robust dynamical balances obtained from the fundamental MHD equations. The linchpin of the approach is the concept of gyroscopic pumping whereby the meridional circulation across isosurfaces of specific angular momentum is linked to the angular momentum transport by the convective Reynolds stress. We find that the amplitude of the convective velocity must be at least 30 m s$^{-1}$ in the upper CZ ($r \sim 0.95 R$) and at least 8 m s$^{-1}$ in the lower CZ ($r \sim 0.75 R$) in order to be consistent with the observed mean flows. Using the base of the near-surface shear layer as a probe of the rotational influence, we are further able to show that the characteristic length scale of deep convective motions must be no smaller than 5.5--30 Mm. These results are compatible with convection models but suggest that the efficiency of the turbulent transport assumed in advection-dominated flux-transport dynamo models is generally not consistent with the mean flows they employ.