Is a deep one-cell meridional circulation essential for the flux transport Solar Dynamo?

TL;DR

This study investigates whether a shallow return flow in the Sun's meridional circulation can sustain the flux transport dynamo, finding that an equatorward flow at the convection zone's bottom is crucial for solar-like magnetic cycle features.

Contribution

The paper demonstrates that a shallow return flow can still produce solar-like dynamo behavior if an equatorward flow exists at the convection zone's base, challenging the traditional one-cell circulation assumption.

Findings

Equatorward flow at the convection zone's bottom is essential.

Low diffusivity leads to unrealistic cycle periods.

Radially downward pumping improves solar-like behavior.

Abstract

The solar activity cycle is successfully modeled by the flux transport dynamo, in which the meridional circulation of the Sun plays an important role. Most of the kinematic dynamo simulations assume a one-cell structure of the meridional circulation within the convection zone, with the equatorward return flow at its bottom. In view of the recent claims that the return flow occurs at a much shallower depth, we explore whether a meridional circulation with such a shallow return flow can still retain the attractive features of the flux transport dynamo (such as a proper butterfly diagram, the proper phase relation between the toroidal and poloidal fields). We consider additional cells of the meridional circulation below the shallow return flow---both the case of multiple cells radially stacked above one another and the case of more complicated cell patterns. As long as there is an equatorward flow in low latitudes at the bottom of the convection zone, we find that the solar behavior is approximately reproduced. However, if there is either no flow or a poleward flow at the bottom of the convection zone, then we cannot reproduce solar behavior. On making the turbulent diffusivity low, we still find periodic behavior, although the period of the cycle becomes unrealistically large. Also, with a low diffusivity, we do not get the observed correlation between the polar field at the sunspot minimum and the strength of the next cycle, which is reproduced when diffusivity is high. On introducing radially downward pumping, we get a more reasonable period and more solar-like behavior even with low diffusivity.