Subcritical dynamo and hysteresis in a Babcock-Leighton type kinematic dynamo model

TL;DR

This study investigates the existence of subcritical dynamos in stars using a Babcock-Leighton kinematic model, revealing hysteresis behavior and the potential for subcritical operation under certain conditions.

Contribution

It demonstrates the possibility of subcritical dynamo behavior and hysteresis in a Babcock-Leighton model, extending previous mean-field and turbulent dynamo findings.

Findings

Hysteresis behavior with two possible dynamo solutions.

Subcritical dynamo can operate depending on initial conditions.

Subcritical mode is unstable under large fluctuations.

Abstract

In Sun and sun-like stars, it is believed that the cycles of the large-scale magnetic field are produced due to the existence of differential rotation and helicity in the plasma flows in their convection zones (CZs). Hence, it is expected that for each star, there is a critical dynamo number for the operation of a large-scale dynamo. As a star slows down, it is expected that the large-scale dynamo ceases to operate above a critical rotation period. In our study, we explore the possibility of the operation of the dynamo in the subcritical region using the Babcock--Leighton type kinematic dynamo model. In some parameter regimes, we find that the dynamo shows hysteresis behavior, i.e., two dynamo solutions are possible depending on the initial parameters -- decaying solution if started with weak field and strong oscillatory solution (subcritical dynamo) when started with a strong field. However, under large fluctuations in the dynamo parameter, the subcritical dynamo mode is unstable in some parameter regimes. Therefore, our study supports the possible existence of subcritical dynamo in some stars which was previously shown in a mean-field dynamo model with distributed $\alpha$ and MHD turbulent dynamo simulations.