Hysteresis between distinct modes of turbulent dynamos

TL;DR

This paper demonstrates through 3D simulations that dynamo hysteresis, previously predicted by mean-field models, can occur in turbulent dynamos, showing bistability and potential links to solar activity modes like grand minima.

Contribution

It provides the first direct numerical simulation evidence of dynamo hysteresis, confirming predictions from mean-field models and exploring its implications for solar activity variability.

Findings

Dynamo hysteresis is observed in 3D simulations.

Bistability depends on initial conditions and parameters.

Hysteresis may relate to solar activity modes like grand minima.

Abstract

Nonlinear mean-field models of the solar dynamo show long-term variability, which may be relevant to different states of activity inferred from long-term radiocarbon data. This paper is aimed to probe the dynamo hysteresis predicted by the recent mean-field models of Kitchatinov \& Olemskoy (2010) with direct numerical simulations. We perform three-dimensional simulations of large-scale dynamos in a shearing box with helically forced turbulence. As initial condition, we either take a weak random magnetic field or we start from a snapshot of an earlier simulation. Two quasi-stable states are found to coexist in a certain range of parameters close to the onset of the large-scale dynamo. The simulations converge to one of these states depending on the initial conditions. When either the fractional helicity or the magnetic Prandtl number is increased between successive runs above the critical value for onset of the dynamo, the field strength jumps to a finite value. However, when the fractional helicity or the magnetic Prandtl number is then decreased again, the field strength stays at a similar value (strong field branch) even below the original onset. We also observe intermittent decaying phases away from the strong field branch close to the point where large-scale dynamo action is just possible. The dynamo hysteresis seen previously in mean-field models is thus reproduced by 3D simulations. Its possible relation to distinct modes of solar activity such as grand minima is discussed.