Minimal roles of solar subsurface meridional flow in the distributed-shear Babcock-Leighton dynamo

¹ School of Space and Earth Sciences, Beihang University, Beijing, China
² Key Laboratory of Space Environment Monitoring and Information Processing of MIIT, Beijing, China

^⋆ Corresponding author: jiejiang@buaa.edu.cn

Received: 11 February 2025
Accepted: 10 July 2025

Abstract

Context. The subsurface meridional flow has long been recognized as a critical factor in driving the solar cycle. Specifically, the equatorward return flow in the tachocline is widely believed to be responsible for the formation of the sunspot butterfly diagram and determine the solar cycle period within the framework of flux transport dynamo (FTD) models.

Aims. We aim to investigate whether the subsurface meridional flow also plays a significant role in the recently developed distributed-shear Babcock-Leighton (BL) dynamo model, which operates within the convection zone, rather than the tachocline.

Methods. Various meridional flow configurations, including a deep single cell, a shallow single cell, and double cells, were applied in the distributed-shear BL dynamo model to explore the mechanisms driving the butterfly diagram and variations in the cycle period.

Results. Subsurface meridional flow plays a minimal role in the distributed-shear BL dynamo. A solar-like butterfly diagram can be generated even with a double-cell meridional flow. The diagram arises from the time- and latitude-dependent regeneration of the toroidal field, governed by latitude-dependent latitudinal differential rotation and the evolution of surface magnetic fields. The cycle period is determined by the surface flux source and transport process responsible for polar field generation, which corresponds to the α-effect in the BL-type dynamo. The cycle period may exhibit varying dependence on the amplitude of the subsurface flow.

Conclusions. The distributed-shear BL dynamo differs fundamentally from the FTD models, as it does not rely on the subsurface flux transport. This distinction aligns the distributed-shear BL dynamo more closely with the original BL dynamo and the conventional αΩ dynamo. Although the subsurface meridional flow plays a negligible role in our distributed-shear BL dynamo, the poleward surface flow is essential.