On the possibility of a bimodal solar dynamo

TL;DR

This paper presents a coupled interface and tachocline dynamo model suggesting the potential for a bimodal solar dynamo, with implications for understanding grand minima in solar activity.

Contribution

It introduces a simple coupled dynamo model with a free parameter fixed by helioseismic data, revealing two steady solutions that may explain solar activity variations.

Findings

Existence of two steady solutions with different tachocline thicknesses.

A second solution with weaker magnetic fields may relate to grand minima.

Model reproduces helioseismic tachocline thickness constraints.

Abstract

A simple way to couple an interface dynamo model to a fast tachocline model is presented, under the assumption that the dynamo saturation is due to a quadratic process and that the effect of finite shear layer thickness on the dynamo wave frequency is analoguous to the effect of finite water depth on surface gravity waves. The model contains one free parameter which is fixed by the requirement that a solution should reproduce the helioseismically determined thickness of the tachocline. In this case it is found that, in addition to this solution, another steady solution exists, characterized by a four times thicker tachocline and 4-5 times weaker magnetic fields. It is tempting to relate the existence of this second solution to the occurrence of grand minima in solar activity.