Convective Dynamo Simulation with a Grand Minimum

TL;DR

This study simulates a Sun-like star's magnetic dynamo, revealing regular polarity reversals, magnetic energy cycles, and a grand minimum phase, driven by complex convection and magnetic interactions.

Contribution

It presents a detailed 3D MHD simulation of stellar dynamo behavior, including grand minima, using an advanced spherical harmonic code with viscosity minimization.

Findings

Recurrent polarity reversals every ~6.2 years.

Magnetic energy exhibits regular cyclic variations.

The star experiences a ~20-year grand minimum phase.

Abstract

The global-scale dynamo action achieved in a simulation of a Sun-like star rotating at thrice the solar rate is assessed. The 3-D MHD Anelastic Spherical Harmonic (ASH) code, augmented with a viscosity minimization scheme, is employed to capture convection and dynamo processes in this G-type star. The simulation is carried out in a spherical shell that encompasses 3.8 density scale heights of the solar convection zone. It is found that dynamo action with a high degree of time variation occurs, with many periodic polarity reversals occurring roughly every 6.2 years. The magnetic energy also rises and falls with a regular period. The magnetic energy cycles arise from a Lorentz-force feedback on the differential rotation, whereas the processes leading to polarity reversals are more complex, appearing to arise from the interaction of convection with the mean toroidal fields. Moreover, an equatorial migration of toroidal field is found, which is linked to the changing differential rotation, and potentially to a nonlinear dynamo wave. This simulation also enters a grand minimum lasting roughly 20~years, after which the dynamo recovers its regular polarity cycles.