Dipole Collapse and Dynamo Waves in Global Direct Numerical Simulations

Abstract

Magnetic fields of low-mass stars and planets are thought to originate from self-excited dynamo action in their convective interiors. Observations reveal a variety of field topologies ranging from large-scale, axial dipole to more structured magnetic fields. In this article, we investigate more than 70 three-dimensional, self-consistent dynamo models obtained by direct numerical simulations. The control parameters, the aspect ratio and the mechanical boundary conditions have been varied to build up this sample of models. Both, strongly dipolar and multipolar models have been obtained. We show that these dynamo regimes can in general be distinguished by the ratio of a typical convective length-scale to the Rossby radius. Models with a predominantly dipolar magnetic field were obtained, if the convective length scale is at least an order of magnitude larger than the Rossby radius. Moreover, we highlight the role of the strong shear associated with the geostrophic zonal flow for models with stress-free boundary conditions. In this case the above transition disappears and is replaced by a region of bistability for which dipolar and multipolar dynamos co-exist. We interpret our results in terms of dynamo eigenmodes using the so-called test field method. We can thus show that models in the dipolar regime are characterized by an isolated 'single mode'. Competing overtones become significant as the boundary to multipolar dynamos is approached. We discuss how these findings relate to previous models and to observations.

The Maunder minimum (1645–1715) was indeed a grand minimum: A reassessment of multiple datasets

“…Charbonneau 2010). Although modern solar-type dynamo models reproduce the "regular" part of the behaviour of the large-scale solar magnetic field (Ghizaru et al 2010;Brown et al 2011;Schrinner et al 2012;Gastine et al 2012;Käpylä et al 2012), it is still challenging to extend the integration time over several magnetic cycles as needed to reproduce a Maunder-like minimum (see, however, Passos & Charbonneau 2014;Augustson et al 2015). Thus, while the MM is identified as a special mode of the solar dynamo, we are not yet able to precisely model it.…”

Rotation and differential rotation of activeKeplerstars

“…Direct numerical simulations showed that models with low Rossby numbers (i.e., fast rotating stars) generate strong dipolar magnetic fields (Schrinner et al 2012). These fields amplify Lorentz forces able to suppress Coriolis forces, and hence can effectively suppress DR (Gastine et al 2012).…”

“…As the depth of the convection zone decreases, the dipolarity breaks down rendering the rotation nonuniform (Schrinner et al 2012). This effect could explain the strong increase of ΔΩ around T eff > 6000 K. Browning (2011) provided an explanation for the shallow increase of α towards cooler stars.…”

The Solar-Stellar Connection

“…high mass and very low mass stars) interested readers may refer to (e.g. Browning et al 2004;Brun et al 2005;Dobler et al 2006;Brown et al 2008;Christensen et al 2009;Featherstone et al 2009;Augustson et al 2011;Cai et al 2011;Morin et al 2011;Schrinner et al 2012, and references therein). Convection plays a key role in transporting the star's heat by having strong correlations between vertical motions and temperature fluctuations.…”

“…The key results can be summarized as follow: 3-D numerical simulations of dynamo action in solar-like stars have revealed a large range of behavior, from steady dynamo, to irregular and cyclic ones Racine et al 2011;Gastine et al 2012;Schrinner et al 2012;Augustson et al 2013;Nelson et al 2013;Schrinner et al 2014, and references therein).…”