On non-axisymmetric magnetic equilibria in stars

Braithwaite, Jonathan

doi:10.1111/j.1365-2966.2008.13218.x

Cited by 124 publications

(177 citation statements)

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“…It was suggested by Prendergast (1956) that a stellar magnetic field in stable axisymmetric equilibrium must contain both poloidal (meridional) and toroidal (azimuthal) components, since both are unstable on their own (Tayler 1973;Wright 1973). This was confirmed recently by numerical simulations by Braithwaite & Spruit (2004); Braithwaite & Nordlund (2006);Braithwaite (2008) who showed that initial stochastic helical fields evolve on an Alfvén timescale Dynamics of fossil magnetic fields in massive star interiors 161 into stable configurations: axisymmetric and non-axisymmetric mixed poloidal-toroidal fields were found. This phenomenon well known in plasma physics is a MHD turbulent relaxation (i.e.…”

Section: Introductionsupporting

confidence: 64%

“…Moreover, as shown in Fig. 7 in Braithwaite (2008), the selective decay of the magnetic helicity (H) and of the magnetic energy (E mag ) assumed here occurs and the transport of flux and mass in the radial direction is inhibited because of the stable stratification and the mass encompassed in poloidal magnetic surfaces is conserved (i.e. M Ψ ).…”

Section: Relaxed Non Force-free Configurationsmentioning

confidence: 99%

“…Let us now compare this analytical configuration to those obtained using numerical simulations (see Braithwaite & Spruit 2004;Braithwaite & Nordlund 2006;Braithwaite 2008). Braithwaite and collaborators performed numerical magnetohydrodynamical simulations of the relaxation of an initially random magnetic field in a stably stratified star.…”

Section: Relaxed Non Force-free Configurationsmentioning

confidence: 99%

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Dynamics of fossil magnetic fields in massive star interiors

Mathis

2010

Proc. IAU

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Abstract. In this talk, I review the different MHD processes, which take place in massive star interiors. First, I describe MHD instabilities, which act on magnetic fields in stellar radiation zones, and the dynamo action in massive stars that give strong indications in favor of a fossil origin of the fields observed at the surface of these stars. Then, I discuss the study of MHD turbulent relaxation processes, which are now examined in stellar interiors, to describe initial conditions for fossil magnetic fields. Finally, I focus on the state of the art of the modeling of the interaction between differential rotation, fossil magnetic field, meridional circulation, and turbulence.

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Section: Introductionsupporting

confidence: 64%

Section: Relaxed Non Force-free Configurationsmentioning

confidence: 99%

Section: Relaxed Non Force-free Configurationsmentioning

confidence: 99%

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Dynamics of fossil magnetic fields in massive star interiors

Mathis

2010

Proc. IAU

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“…Often, but not always (Braithwaite 2008), these are roughly axisymmetric combinations of linked poloidal and toroidal components, whose external appearance is essentially dipolar. It appears plausible that these configurations approximate the true magnetic field structures in upper main sequence stars, white dwarfs, and neutron stars.…”

Section: Introductionmentioning

confidence: 99%

Stable magnetic equilibria and their evolution in the upper main sequence, white dwarfs, and neutron stars

Reisenegger

2009

A&A

114

141

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Context. Long-lived, large-scale magnetic field configurations exist in upper main sequence, white dwarf, and neutron stars. Externally, these fields have a strong dipolar component, while their internal structure and evolution are uncertain but highly relevant to several problems in stellar and high-energy astrophysics. Aims. We discuss the main properties expected for the stable magnetic configurations in these stars from physical arguments and the ways these properties may determine the modes of decay of these configurations. Methods. We explain and emphasize the likely importance of the non-barotropic, stable stratification of matter in all these stars (due to entropy gradients in main-sequence envelopes and white dwarfs, due to composition gradients in neutron stars). We first illustrate it in a toy model involving a single, azimuthal magnetic flux tube. We then discuss the effect of stable stratification or its absence on more general configurations, such as axisymmetric equilibria involving poloidal and toroidal field components. We argue that the main mode of decay for these configurations are processes that lift the constraints set by stable stratification, such as heat diffusion in main-sequence envelopes and white dwarfs, and beta decays or particle diffusion in neutron stars. We estimate the time scales for these processes, as well as their interplay with the cooling processes in the case of neutron stars. Results. Stable magneto-hydrostatic equilibria appear to exist in stars whenever the matter in their interior is stably stratified (not barotropic). These equilibria are not force-free and not required to satisfy the Grad-Shafranov equation, but they do involve both toroidal and poloidal field components. In main sequence stars with radiative envelopes and in white dwarfs, heat diffusion is not fast enough to make these equilibria evolve over the stellar lifetime. In neutron stars, a strong enough field might decay by overcoming the compositional stratification through beta decays (at the highest field strengths) or through ambipolar diffusion (for somewhat weaker fields). These processes convert magnetic energy to thermal energy, and they occur at significant rates only once the latter is less than the former; therefore, they substantially delay the cooling of the neutron star, while slowly decreasing its magnetic energy.

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“…One possibility is that, at the end of the MS and its associated dynamo, the magnetic field is able to evolve into a stable configuration only in ≈50% of the cases. The evolution of magnetic fields into stable magnetic configurations has been studied (Braithwaite & Nordlund 2006), but it is difficult to make detailed predictions as the outcome depends on the complex initial conditions of the magnetic configuration left by turbulent convection, in particular magnetic energy and helicity (Braithwaite 2008). Moreover, these theoretical calculations do not include a compositional stratification, which instead might play an important role in confining the magnetic field after dynamo action ceases in the convective cores of stars.…”

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confidence: 99%

Asteroseismic Signatures of Evolving Internal Stellar Magnetic Fields

Cantiello

Fuller

Bildsten

2016

ApJ

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Recent asteroseismic analyses indicate the presence of strong (B  10 5 G) magnetic fields in the cores of many red giant stars. Here, we examine the implications of these results for the evolution of stellar magnetic fields, and we make predictions for future observations. Those stars with suppressed dipole modes indicative of strong core fields should exhibit moderate but detectable quadrupole mode suppression. The long magnetic diffusion times within stellar cores ensure that dynamo-generated fields are confined to mass coordinates within the main-sequence (MS) convective core, and the observed sharp increase in dipole mode suppression rates above 1.5 M e is likely explained by the larger convective core masses and faster rotation of these more massive stars. In clump stars, core fields of ∼10 5 G can suppress dipole modes, whose visibility should be equal to or less than the visibility of suppressed modes in ascending red giants. High dipole mode suppression rates in low-mass (M  2 M e ) clump stars would indicate that magnetic fields generated during the MS can withstand subsequent convective phases and survive into the compact remnant phase. Finally, we discuss implications for observed magnetic fields in white dwarfs and neutron stars, as well as the effects of magnetic fields in various types of pulsating stars.

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On non-axisymmetric magnetic equilibria in stars

Cited by 124 publications

References 38 publications

Dynamics of fossil magnetic fields in massive star interiors

Dynamics of fossil magnetic fields in massive star interiors

Stable magnetic equilibria and their evolution in the upper main sequence, white dwarfs, and neutron stars

Asteroseismic Signatures of Evolving Internal Stellar Magnetic Fields

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