R J Tayler

Bio: R J Tayler is an academic researcher. The author has contributed to research in topics: Pinch. The author has an hindex of 1, co-authored 1 publications receiving 13 citations.

Convective instability in the presence of a nonuniform horizontal magnetic field

Abstract: Newcomb’s criterion for convective stability in the presence of a horizontal magnetic field is written in a form which explicitly shows the effect of vertical variations of the magnetic field strength. It is shown that a nonuniform horizontal magnetic field can be destabilizing as well as stabilizing.

Hydrodynamic and hydromagnetic stability of the Couette flow

Abstract: The stability of the dissipative Couette flow is examined in the linear approximation. The onset conditions and flow instability properties are studied, with special emphasis placed on the instability properties in the presence of a magnetic field and density stratification. Theoretical and experimental results on instability parameters are found to be in good agreement (within a few percent).

Stratorotational instability in MHD Taylor-Couette flows

Abstract: Aims. The stability of the dissipative Taylor-Couette flow with a stable axial density stratification and a prescribed azimuthal magnetic field is considered. Methods. Global nonaxisymmetric solutions of the linearized MHD equations with toroidal magnetic field, density stratification, and differential rotation are found for both insulating and conducting cylinders. Results. Hydrodynamic calculations for various gap widths show that flat rotation laws such as the Kepler rotation are always unstable against SRI. Quasigalactic rotation laws, however, are stable for wide gaps. The influence of a current-free toroidal magnetic field on SRI strongly depends on the magnetic Prandtl number Pm: SRI is supported by Pm > 1 and it is suppressed by Pm < 1. For rotation laws that are too flat, when the hydrodynamic SRI ceases, a smooth transition exists to the instability that the toroidal magnetic field produces in combination with the differential rotation. For the first time this nonaxisymmetric azimuthal magnetorotational instability (AMRI) has been computed in the presence of an axial density gradient. If the magnetic field between the cylinders is not current-free, then the Tayler instability occurs, too. The transition from the nonmagnetic centrifugal instability to the magnetic Tayler instability in the presence of differential rotation and density stratification proves to be complex. Most spectacular is the “ballooning” of the stability domain by the density stratification: already a small rotation stabilizes magnetic fields against the Tayler instability.

Magnetic inhibition of convection in O-star envelopes

Abstract: It has been suggested that the absence of macroturbulence in the atmosphere of NGC 1624 - 2 is due its strong magnetic field (the strongest known for a massive O star) suppressing convection in its outer layers, removing the mechanism thought responsible for the observed macroturbulence in stars with lower field strengths. Here, we develop and apply a criterion for a uniform magnetic field to suppress convection in stellar envelopes in which radiation pressure is a significant contributor to hydrostatic balance. We find upper mass limits of ~55 Msun and ~30 Msun for magnetic suppression to be possible in Zero Age Main Sequence and Terminal Age Main Sequence stars, respectively. For evolved stars, magnetic suppression of convection can significantly alter the stars' evolution. For NGC 1624 - 2, we find a polar dipole strength of 16.5 +/- 5.9 kG is required to suppress convection, in good agreement with the value ~20 kG measured by spectropolarimetry.

Stratorotational instability in MHD Taylor-Couette flows

Abstract: The stability of dissipative Taylor-Couette flows with an axial stable density stratification and a prescribed azimuthal magnetic field is considered. Global nonaxisymmetric solutions of the linearized MHD equations with toroidal magnetic field, axial density stratification and differential rotation are found for both insulating and conducting cylinder walls. Flat rotation laws such as the quasi-Kepler law are unstable against the nonaxisymmetric stratorotational instability (SRI). The influence of a current-free toroidal magnetic field depends on the magnetic Prandtl number Pm: SRI is supported by Pm > 1 and it is suppressed by Pm \lsim 1. For too flat rotation laws a smooth transition exists to the instability which the toroidal magnetic field produces in combination with the differential rotation. This nonaxisymmetric azimuthal magnetorotational instability (AMRI) has been computed under the presence of an axial density gradient. If the magnetic field between the cylinders is not current-free then also the Tayler instability occurs and the transition from the hydrodynamic SRI to the magnetic Tayler instability proves to be rather complex. Most spectacular is the `ballooning' of the stability domain by the density stratification: already a rather small rotation stabilizes magnetic fields against the Tayler instability. An azimuthal component of the resulting electromotive force only exists for density-stratified flows. The related alpha-effect for magnetic SRI of Kepler rotation appears to be positive for negative d\rho/dz <0.