On the Amplitude of Convective Velocities in the Deep Solar Interior

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 ($r\sim 0.95R$) and at least 8 m s${}^{-1}$ in the lower CZ ($r\sim 0.75R$) 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.

16 pages, 4 figures, accepted to the Astrophysical Journal

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• Forecasting the solar activity cycle: new insights