Flux-transport and mean-field dynamo theories of solar cycles

TL;DR

This paper discusses the use of flux transport and mean-field dynamo models, especially the high-diffusivity flux transport dynamo, to explain solar cycle features and irregularities, emphasizing the role of meridional circulation and Babcock--Leighton mechanism.

Contribution

It advocates for the flux transport dynamo model as the best current theoretical framework for understanding solar cycles, highlighting the importance of high diffusivity and circulation fluctuations.

Findings

High-diffusivity flux transport dynamo aligns with observational data.

Cycle irregularities mainly stem from fluctuations in the Babcock--Leighton mechanism.

Meridional circulation fluctuations explain phenomena like the Waldmeier effect.

Abstract

We point out the difficulties in carrying out direct numerical simulation of the solar dynamo problem and argue that kinematic mean-field models are our best theoretical tools at present for explaining various aspects of the solar cycle in detail. The most promising kinematic mean-field model is the flux transport dynamo model, in which the toroidal field is produced by differential rotation in the tachocline, the poloidal field is produced by the Babcock--Leighton mechanism at the solar surface and the meridional circulation plays a crucial role. Depending on whether the diffusivity is high or low, either the diffusivity or the meridional circulations provides the main transport mechanism for the poloidal field to reach the bottom of the convection zone from the top. We point out that the high-diffusivity flux transport dynamo model is consistent with various aspects of observational data. The irregularities of the solar cycle are primarily produced by fluctuations in the Babcock--Leighton mechanism and in the meridional circulation. We summarize recent work on the fluctuations of meridional circulation in the flux transport dynamo, leading to explanations of such things as the Waldmeier effect.