Cycling Dynamo in a Young Sun: Grand Minima and Equatorward Propagation

TL;DR

This study simulates a young Sun's magnetic dynamo, revealing regular polarity reversals, equatorward magnetic field migration, and a grand minimum phase, driven by complex feedback mechanisms in stellar convection.

Contribution

First detailed 3D MHD simulation of a young Sun's dynamo showing polarity cycles, equatorial migration, and grand minima, advancing understanding of stellar magnetic activity.

Findings

Dynamo exhibits polarity reversals every 6.2 years.

Magnetic energy cycles are synchronized with polarity reversals.

Simulation shows a 20-year grand minimum period.

Abstract

We assess the global-scale dynamo action achieved in a simulation of a sun-like star rotating at three times the solar rate. The 3-D MHD Anelastic Spherical Harmonic code, using slope-limited diffusion, is employed to capture convection and dynamo processes in such a young sun. The simulation is carried out in a spherical shell that encompasses 3.8 density scale heights of the solar convection zone. We find that dynamo action with a high degree of time variation occurs, with many periodic polarity reversals every 6.2 years. The magnetic energy also rises and falls with a regular period, with two magnetic energy cycles required to complete a polarity cycle. These magnetic energy cycles arise from a Lorentz-force feedback on the differential rotation, whereas the polarity reversals are present due to the spatial separation of the equatorial and polar dynamos. Moreover, an equatorial migration of toroidal field is found, which is linked to the changing differential rotation and to a near-surface shear layer. This simulation also enters a grand minimum lasting roughly 20 years, after which the dynamo recovers its regular polarity cycles.