Hysteresis near the transition of the large-scale dynamo in the presence of the small-scale dynamo

TL;DR

This study investigates the coexistence and hysteresis behavior of large-scale and small-scale dynamos in stellar contexts, demonstrating the existence of a sub-critical large-scale dynamo branch influenced by initial conditions.

Contribution

It provides numerical evidence for hysteresis and sub-critical large-scale dynamo operation in the presence of small-scale dynamo effects, extending previous models.

Findings

Hysteresis behavior observed in dynamo solutions.

Sub-critical large-scale dynamo branch confirmed.

Hysteresis regime is narrower with small-scale dynamo presence.

Abstract

In Sun and solar-type stars, there is a critical dynamo number for the operation of a large-scale dynamo, below which the dynamo ceases to operate. This region is known as the subcritical region. Previous studies showed the possibility of operating the solar-like large-scale (global) dynamo in the subcritical region without a small-scale dynamo. As in the solar convection zone, both large- and small-scale dynamos are expected to operate at the same time and location, we check the robustness of the previously identified subcritical dynamo branch in a numerical model in which both large- and small-scale dynamos are excited. For this, we use the {\sc Pencil Code} and set up an $\alpha\Omega$ dynamo model with uniform shear and helically forced turbulence. We have performed a few sets of simulations at different relative helicity to explore the generation of large-scale oscillatory fields in the presence of small-scale dynamo. We find that in some parameter regimes, the dynamo shows hysteresis behavior, i.e., two dynamo solutions are possible depending on the initial parameters used. A decaying solution when the dynamo was started with a weak field and a strong oscillatory solution if the dynamo was initialized with a strong field. Thus, the existence of the sub-critical branch of the large-scale dynamo in the presence of small-scale dynamo is established. However, the regime of hysteresis is quite narrow with respect to the case without the small-scale dynamo. Our work supports the possible existence of large-scale dynamo in the sub-critical regime of slowly rotating stars.