Studies of grand minima in sunspot cycles from a flux transport solar dynamo model

TL;DR

This study uses a flux transport dynamo model to investigate the causes and characteristics of grand minima and maxima in solar activity, highlighting the roles of meridional circulation fluctuations and the Babcock-Leighton mechanism.

Contribution

It demonstrates the effectiveness of meridional circulation fluctuations in producing grand minima and explores how the Babcock-Leighton mechanism can operate during these periods.

Findings

Fluctuations in meridional circulation are more effective in causing grand minima.

Durations and waiting times of grand minima and maxima follow exponential distributions.

A fine-tuned alpha-effect can reactivate the dynamo during grand minima.

Abstract

We propose that grand minima in solar activity are caused by simultaneous fluctuations in the meridional circulation and the Babcock-Leighton mechanism for the poloidal field generation in the flux transport dynamo model. We present the following results: (a) fluctuations in the meridional circulation are more effective in producing grand minima; (b) both sudden and gradual initiations of grand minima are possible; (c) distributions of durations and waiting times between grand minima seem to be exponential; (d) the coherence time of the meridional circulation has an effect on the number and the average duration of grand minima, a coherence time of about 30 years being consistent with observational data. We also study the occurrence of grand maxima and find that the distributions of durations and waiting times between grand maxima are also exponential, like the grand minima. Finally we address the question whether the Babcock-Leighton mechanism can be operative during grand minima when there are no sunspots. We show that an alpha-effect restricted to the upper portions of the convection zone can pull the dynamo out of the grand minima and can match various observational requirements if the amplitude of this alpha-effect is suitably fine-tuned.