Mechanisms of Planetary and Stellar Dynamos

We review some of the recent progress on modeling planetary and stellar dynamos. Particular attention is given to the dynamo mechanisms and the resulting properties of the field. We present direct numerical simulations using a simple Boussinesq model. These simulations are interpreted using the classical mean-field formalism. We investigate the transition from steady dipolar to multipolar dynamo waves solutions varying different control parameters, and discuss the relevance to stellar magnetic fields. We show that owing to the role of the strong zonal flow, this transition is hysteretic. In the presence of stress-free boundary conditions, the bistability extends over a wide range of parameters.Comment: Proceedings of IAUS 294 "Solar and Astrophysical Dynamos and Magnetic Activity" Editors A.G. Kosovichev, E.M. de Gouveia Dal Pino, & Y.Yan, Cambridge University Press, to appear (2013

Dipole Collapse and Dynamo Waves in Global Direct Numerical Simulations

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.Comment: 35 pages, 16 figure

Subcritical transition to turbulence triggered by a magnetic dynamo

It has recently been shown that a significant slowdown of many stars can be attributed to the emergence of a strong magnetic field within the radiative region, where heat is transferred through radiation in a stably stratified layer. Here, we describe how this transition can be understood as a subcritical bifurcation to small-scale turbulence in linearly stable flows. The turbulence is sustained by a nonlinear mean-field dynamo and can be observed down to relatively small differential rotation, arbitrarily far from the linear onset of any hydrodynamic instability. In this regime, turbulent fluctuations provide diffusivity-free transfer of angular momentum that increases the transport generated by the magnetic field triggering the turbulence. Finally, we present a simple nonlinear model that captures this scenario and can be used as a general description of the transition to turbulence in astrophysical flows, as long as it involves a competition between a large-scale dynamo, and a small-scale magnetic instability.Comment: 17 pages, 9 figures, accepted in Phys. Rev. Fluid

Tayler-Spruit dynamos in simulated radiative stellar layers

The Tayler-Spruit dynamo mechanism has been proposed two decades ago as a plausible mechanism to transport angular momentum in radiative stellar layers. Direct numerical simulations are still needed to understand its trigger conditions and the saturation mechanisms. The present study follows up on (Petitdemange et al. 2023), where we reported the first numerical simulations of a Tayler-Spruit dynamo cycle. Here we extend the explored parameter space to assess in particular the influence of stratification on the dynamo solutions. We also present numerical verification of theoretical assumptions made in (Spruit 2002), which are instrumental in deriving the classical prescription for angular momentum transport implemented in stellar evolution codes. A simplified radiative layer is modeled numerically by considering the dynamics of a stably-stratified, differentially rotating, magnetized fluid in a spherical shell. Our simulations display a diversity of magnetic field topologies and amplitudes depending on the flow parameters, including hemispherical solutions. The Tayler-Spruit dynamos reported here are found to satisfy magnetostrophic equilibrium and achieve efficient turbulent transport of angular momentum, following Spruit's heuristic prediction

Magnetostrophic MRI in the Earth's Outer Core

We show that a simple, modified version of the Magnetorotational Instability (MRI) can develop in the outer liquid core of the Earth, in the presence of a background shear. It requires either thermal wind, or a primary instability, such as convection, to drive a weak differential rotation within the core. The force balance in the Earth's core is very unlike classical astrophysical applications of the MRI (such as gaseous disks around stars). Here, the weak differential rotation in the Earth core yields an instability by its constructive interaction with the planet's much larger rotation rate. The resulting destabilising mechanism is just strong enough to counteract stabilizing resistive effects, and produce growth on geophysically interesting timescales. We give a simple physical explanation of the instability, and show that it relies on a force balance appropriate to the Earth's core, known as magnetostrophic balance

Two-dimensional non-linear simulations of the magnetostrophic magnetorotational instability

International audienceWe have shown that a simple, modified version of the Magnetorotational Instability (MRI) can, in principle, develop in the Earth's outer liquid core in the presence of a background shear (see Petitdemange, Dormy and Balbus, MagnetoStrophic MRI in the Earth's outer core. Geophys. Res. Lett. 2008, 35 15305). We refer to this instability as the Magnetostrophic MRI (MS-MRI). In this article, we extend our investigations to the nonlinear regime and present results from global axisymmetric simulations in spherical geometry. We show that as the angular momentum is transported outward, the MS-MRI saturates by rapidly changing the initial shear profile. Therefore, the saturation process differs substantially from traditional MRI applications (e.g. accretion disks) in which the background shear is essentially fixed. We show that the MS-MRI appears as a new constraint which limits the maximum differential rotation. To illustrate this mechanism, we apply this work to a Jupiter-like planet, and argue that the magnetic field eventually destabilises the conducting zone of this planet. According to these results, purely hydrodynamic models for the deep origin of the banded structure of Jupiter may need to be modified

Systematic parameter study of dynamo bifurcations in geodynamo simulations

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A magnetically driven equatorial jet in Europa’s ocean

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