Magnetic fields can be created in stably stratified (non-convective) layers in a differentially rotating star. A magnetic instability in the toroidal field (wound up by differential rotation) replaces the role of convection in closing the field amplification loop. Tayler instability is likely to be the most relevant magnetic instability. A dynamo model is developed from these ingredients, and applied to the problem of angular momentum transport in stellar interiors. It produces a predominantly horizontal field. This dynamo process is found to be more effective in transporting angular momentum than the known hydrodynamic mechanisms. It might account for the observed pattern of rotation in the solar core.

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Dynamo action by differential rotation in a stably stratified stellar interior

Some main-sequence stars of spectral type A are observed to have a strong (0.03–3 tesla), static, large-scale magnetic field, of a chiefly dipolar shape: they are known as ‘Ap stars’1,2,3,4, such as Alioth, the fifth star in the Big Dipper. Following the discovery of these fields, it was proposed that they are remnants of the star's formation, a ‘fossil’ field5,6. An alternative suggestion is that they could be generated by a dynamo process in the star's convective core7. The dynamo hypothesis, however, has difficulty explaining high field strengths and the observed lack of a correlation with rotation. The weakness of the fossil-field theory has been the absence of field configurations stable enough to survive in a star over its lifetime. Here we report numerical simulations that show that stable magnetic field configurations, with properties agreeing with those observed, can develop through evolution from arbitrary, unstable initial fields. The results are applicable equally to Ap stars, magnetic white dwarfs and some highly magnetized neutron stars known as magnetars. This establishes fossil fields as the natural, unifying explanation for the magnetism of all these stars.

https://arxiv.org/pdf/astro-ph/0502043.pdf

A fossil origin for the magnetic field in A stars and white dwarfs

We investigate the 50-year old hypothesis that the magnetic fields of the Ap stars are stable equilibria that have survived in these stars since their formation. With numerical simulations we find that stable magnetic field configurations indeed appear to exist under the conditions in the radiative interior of a star. Confirming a hypothesis by Prendergast (1956, ApJ, 123, 498), the configurations have roughly equal poloidal and toroidal field strengths. We find that tori of such twisted fields can form as remnants of the decay of an unstable random initial field. In agreement with observations, the appearance at the surface is an approximate dipole with smaller contributions from higher multipoles, and the surface field strength can increase with the age of the star. The results of this paper were summarised by Braithwaite & Spruit (2004, Nature, 431, 891).

/pdf/stable-magnetic-fields-in-stellar-interiors-4xk2ahcu0j.pdf

Stable magnetic fields in stellar interiors

In this third paper in a series on stable magnetic equilibria in stars, I look at the stability of axisymmetric field configurations and, in particular, the relative strengths of the toroidal and poloidal components. Both toroidal and poloidal fields are unstable on their own, and stability is achieved by adding the two together in some ratio. I use Tayler's stability conditions for toroidal fields and other analytic tools to predict the range of stable ratios and then check these predictions by running numerical simulations. If the energy in the poloidal component as a fraction of the total magnetic energy is written as E p /E, it is found that the stability condition is a(E/U) < Ep/E ≤ 0.8 where E/U is the ratio of magnetic to gravitational energy in the star and a is some dimensionless factor whose value is of order 10 in a main-sequence star and of order 10 3 in a neutron star. In other words, whilst the poloidal component cannot be significantly stronger than the toroidal, the toroidal field can be very much stronger than the poloidal-given that in realistic stars we expect E/U < 10- 6 . The implications of this result are discussed in various contexts such as the emission of gravitational waves by neutron stars, free precession and a 'hidden' energy source for magnetars.

/pdf/axisymmetric-magnetic-fields-in-stars-relative-strengths-of-1v4cn1zky7.pdf

Axisymmetric magnetic fields in stars: relative strengths of poloidal and toroidal components

The Mars Exploration Rover Opportunity identified the ferric sulphate mineral jarosite and possible relicts of gypsum at the Meridiani Planum landing site1. On Earth, jarosite has been found to form in acid mine drainage environments, during the oxidation of sulphide minerals2, and during alteration of volcanic rocks by acidic, sulphur-rich fluids near volcanic vents3. Jarosite formation is thus thought to require a wet, oxidizing and acidic environment. But jarosite on Earth only persists over geologically relevant time periods in arid environments because it rapidly decomposes to produce ferric oxyhydroxides in more humid climates4. Here we present equilibrium thermodynamic reaction-path simulations that constrain the range of possible conditions under which such aqueous alteration phases are likely to have formed on Mars. These calculations simulate the chemical weathering of basalt at relevant martian conditions. We conclude that the presence of jarosite combined with residual basalt at Meridiani Planum indicates that the alteration process did not proceed to completion, and that following jarosite formation, arid conditions must have prevailed.

Jarosite as an indicator of water-limited chemical weathering on Mars

/pdf/the-adiabatic-stability-of-stars-containing-magnetic-fields-3rqyu2n8q3.pdf

R. J. Tayler

Papers

The Adiabatic Stability of Stars Containing Magnetic Fields – III: ADDITIONAL RESULTS FOR POLOIDAL FIELDS