Dynamo generated by the centrifugal instability

Marcotte, Florence; Gissinger, Christophe

doi:10.1103/physrevfluids.1.063602

Cited by 10 publications

(9 citation statements)

References 37 publications

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“…Another hypothesis relies on a dynamo action in the radiative envelope. Indeed, differential rotation can trigger various instabilities which lead to dynamo action, as shown by self-consistent numerical simulations (MacDonald & Mullan 2004;Guervilly & Cardin 2010;Arlt & Rüdiger 2011b;Marcotte & Gissinger 2016). Several instabilities are likely to occur in stellar interiors (Spruit 1999).…”

Section: Proposed Mechanisms In Hot Starsmentioning

confidence: 97%

Magnetic fields driven by tidal mixing in radiative stars

Vidal

Cébron

Schaeffer

et al. 2018

Monthly Notices of the Royal Astronomical Society

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Stellar magnetism plays an important role in stellar evolution theory. Approximatively 10 % of observed main sequence (MS) and pre-main-sequence (PMS) radiative stars exhibit surface magnetic fields above the detection limit, raising the question of their origin. These stars host outer radiative envelopes, which are stably stratified. Therefore, they are assumed to be motionless in standard models of stellar structure and evolution. We focus on rapidly rotating, radiative stars which may be prone to the tidal instability, due to an orbital companion. Using direct numerical simulations in a sphere, we study the interplay between a stable stratification and the tidal instability, and assess its dynamo capability. We show that the tidal instability is triggered regardless of the strength of the stratification (Brunt-Väisälä frequency). Furthermore, the tidal instability can lead to both mixing and self-induced magnetic fields in stably stratified layers (provided that the Brunt-Väisälä frequency does not exceed the stellar spin rate in the simulations too much). The application to stars suggests that the resulting magnetic fields could be observable at the stellar surfaces. Indeed, we expect magnetic field strengths up to several Gauss. Consequently, tidally driven dynamos should be considered as a (complementary) dynamo mechanism, possibly operating in radiative MS and PMS stars hosting orbital companions. In particular, tidally driven dynamos may explain the observed magnetism of tidally deformed and rapidly rotating Vega-like stars.

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Section: Proposed Mechanisms In Hot Starsmentioning

confidence: 97%

Magnetic fields driven by tidal mixing in radiative stars

Vidal

Cébron

Schaeffer

et al. 2018

Monthly Notices of the Royal Astronomical Society

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“…Note that these subcritical dynamos are fundamentally different from dynamos driven by centrifugal instability in cylindrical or spherical Couette flow apparatuses (Willis & Barenghi 2002 a ; Nore et al. 2012; Marcotte & Gissinger 2016). More recently, Guseva et al.…”

Section: Introductionmentioning

confidence: 96%

“…Towards this end, the first invariant nonlinear dynamo solution without any external magnetic field was found in MHD rotating plane Couette flow by Rincon, Ogilvie & Proctor (2007), and their results were subsequently extended to a rotating shearing box (see Riols et al 2013). Note that these subcritical dynamos are fundamentally different from dynamos driven by centrifugal instability in cylindrical or spherical Couette flow apparatuses (Willis & Barenghi 2002a;Nore et al 2012;Marcotte & Gissinger 2016). More recently, Guseva et al (2017) succeeded in generating dynamos in a quasi-Keplerian cylindrical Couette flow, motivated by the rotating plane Couette flow and shearing box studies.…”

Section: Introductionmentioning

confidence: 99%

High-speed shear-driven dynamos. Part 2. Numerical analysis

Deguchi

2019

J. Fluid Mech.

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This paper aims to numerically verify the large Reynolds number asymptotic theory of magneto-hydrodynamic (MHD) flows proposed in the companion paper Deguchi (2019). To avoid any complexity associated with the chaotic nature of turbulence and flow geometry, nonlinear steady solutions of the viscous-resistive magneto-hydrodynamic equations in plane Couette flow have been utilized. Two classes of nonlinear MHD states, which convert kinematic energy to magnetic energy effectively, have been determined. The first class of nonlinear states can be obtained when a small spanwise uniform magnetic field is applied to the known hydrodynamic solution branch of the plane Couette flow. The nonlinear states are characterised by the hydrodynamic/magnetic roll-streak and the resonant layer at which strong vorticity and current sheets are observed. These flow features, and the induced strong streamwise magnetic field, are fully consistent with the vortex/Alfvén wave interaction theory proposed in Deguchi (2019). When the spanwise uniform magnetic field is switched off, the solutions become purely hydrodynamic. However, the second class of 'self-sustained shear driven dynamos' at the zero-external magnetic field limit can be found by homotopy via the forced states subject to a spanwise uniform current field. The discovery of the dynamo states has motivated the corresponding large Reynolds number matched asymptotic analysis in Deguchi (2019). Here, the reduced equations derived by the asymptotic theory have been solved numerically. The asymptotic solution provides remarkably good predictions for the finite Reynolds number dynamo solutions. arXiv:1809.03855v2 [physics.flu-dyn]

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“…If such an instability existed, not only would it possibly explain the minimum field of Ap/Bp stars, but it could also be at the origin of dynamo action in the radiative zones of Vega-like stars. Various studies have indeed recently focused on the appealing idea that dynamo action would not require convective motions but, rather, non-axisymmetric hydro or MHD instabilities which, in conjunction with the differential rotation, would produce the electromotive force needed to close the dynamo loop (Spruit 2002;Braithwaite 2006;Zahn et al 2007;Guervilly & Cardin 2010;Marcotte & Gissinger 2016). For now, it is still debated whether such a radiative zone dynamo could exist in stars.…”

Section: Introductionmentioning

confidence: 99%

Interplay between magnetic fields and differential rotation in a stably stratified stellar radiative zone

Jouve

Lignières

Gaurat

2020

A&A

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Context. The interactions between magnetic fields and differential rotation in stellar radiative interiors could play a major role in achieving an understanding of the magnetism of intermediate-mass and massive stars and of the differential rotation profile observed in red-giant stars. Aims. The present study is aimed at studying the flow and field produced by a stellar radiative zone which is initially made to rotate differentially in the presence of a large-scale poloidal magnetic field threading the whole domain. We focus both on the axisymmetric configurations produced by the initial winding-up of the magnetic field lines and on the possible instabilities of those configurations. We investigate in detail the effects of the stable stratification and thermal diffusion and we aim, in particular, to assess the role of the stratification at stabilising the system. Methods. We performed 2D and 3D global Boussinesq numerical simulations started from an initial radial or cylindrical differential rotation and a large-scale poloidal magnetic field. Under the conditions of a large rotation frequency compared to the Alfvén frequency, we built a magnetic configuration strongly dominated by its toroidal component. We then perturbed this configuration to observe the development of non-axisymmetric instabilities. Results. The parameters of the simulations were chosen to respect the ordering of time scales of a typical stellar radiative zone. In this framework, the axisymmetric evolution is studied by varying the relative effects of the thermal diffusion, the Brunt-Väisälä frequency, the rotation, and the initial poloidal field strength. After a transient time and using a suitable adimensionalisation, we find that the axisymmetric state only depends on tes/tAp the ratio between the Eddington–Sweet circulation time scale and the Alfvén time scale. A scale analysis of the Boussinesq magnetohydrodynamical equations allows us to recover this result. In the cylindrical case, a magneto-rotational instability develops when the thermal diffusivity is sufficiently high to enable the favored wavenumbers to be insensitive to the effects of the stable stratification. In the radial case, the magneto-rotational instability is driven by the latitudinal shear created by the back-reaction of the Lorentz force on the flow. Increasing the level of stratification then leaves the growth rate of the instability mainly unaffected while its horizontal length scale grows. Conclusions. Non-axisymmetric instabilities are likely to exist in stellar radiative zones despite the stable stratification. They could be at the origin of the magnetic dichotomy observed in intermediate-mass and massive stars. They are also unavoidable candidates for the transport of angular momentum in red giant stars.

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Dynamo generated by the centrifugal instability

Cited by 10 publications

References 37 publications

Magnetic fields driven by tidal mixing in radiative stars

Magnetic fields driven by tidal mixing in radiative stars

High-speed shear-driven dynamos. Part 2. Numerical analysis

Interplay between magnetic fields and differential rotation in a stably stratified stellar radiative zone

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