Dynamo models of grand minima

TL;DR

This paper explores physical processes behind solar grand minima, emphasizing that fluctuations in poloidal field and meridional circulation, rather than nonlinear effects, may trigger these extended periods of low solar activity.

Contribution

It analyzes potential sources of irregularities in solar dynamo models, highlighting the roles of poloidal field fluctuations and meridional circulation variations in causing grand minima.

Findings

Nonlinear effects are unlikely causes of grand minima.

Fluctuations in poloidal field generation can trigger grand minima.

Weaker meridional circulation may lead to grand minima.

Abstract

Since a universally accepted dynamo model of grand minima does not exist at the present time, we concentrate on the physical processes which may be behind the grand minima. After summarizing the relevant observational data, we make the point that, while the usual sources of irregularities of solar cycles may be sufficient to cause a grand minimum, the solar dynamo has to operate somewhat differently from the normal to bring the Sun out of the grand minimum. We then consider three possible sources of irregularities in the solar dynamo: (i) nonlinear effects; (ii) fluctuations in the poloidal field generation process; (iii) fluctuations in the meridional circulation. We conclude that (i) is unlikely to be the cause behind grand minima, but a combination of (ii) and (iii) may cause them. If fluctuations make the poloidal field fall much below the average or make the meridional circulation significantly weaker, then the Sun may be pushed into a grand minimum.