Field Generation In Stars Research Articles

Magnetic Field Generation in Stars

Enormous progress has been made on observing stellar magnetism in stars from the main sequence through to compact objects. Recent data have thrown into sharper relief the vexed question of the origin of stellar magnetic fields, which remains one of the main unanswered questions in astrophysics. In this chapter we review recent work in this area of research. In particular, we look at the fossil field hypothesis which links magnetism in compact stars to magnetism in main sequence and pre-main sequence stars and we consider why its feasibility has now been questioned particularly in the context of highly magnetic white dwarfs. We also review the fossil versus dynamo debate in the context of neutron stars and the roles played by key physical processes such as buoyancy, helicity, and superfluid turbulence,in the generation and stability of neutron star fields. Independent information on the internal magnetic field of neutron stars will come from future gravitational wave detections. Thus we maybe at the dawn of a new era of exciting discoveries in compact star magnetism driven by the opening of a new, non-electromagnetic observational window. We also review recent advances in the theory and computation of magnetohydrodynamic turbulence as it applies to stellar magnetism and dynamo theory. These advances offer insight into the action of stellar dynamos as well as processes whichcontrol the diffusive magnetic flux transport in stars.

Numerical simulations of stellar convective dynamos. I. the model and method

A numerical model used to simulate global convection and magnetic field generation in stars is described. Nonlinear, three-dimensional, time-dependent solutions of the anelastic magnetohydrodynamic equations are presented for a stratified, rotating, spherical, fluid shell heated from below. The velocity, magnetic field, and thermodynamic perturbations are expanded in spherical harmonics to resolve their horizontal structure and in Chebyshev polynomials to resolve their radial structure. An explicit Adams-Bashforth time integration scheme is used with an implicit Crank-Nicolson treatment of the diffusion terms. Nonlinear terms are computed in physical space; and spatial derivatives are computed in spectral space. The resulting second-order differential equations are solved with a Chebyshev collocation method. Preliminary solutions for a Solar-like model are briefly discussed. Convective motions driven in the upper, superadiabatic part of the zone penetrate into the lower, subadiabatic part of the zone. Differential rotation, induced by the interaction of convection and rotation, is an equatorial acceleration at the surface as observed on the Sun; below the surface angular velocity decreases with depth. The meridional circulation and the equator-pole temperature excess are within Solar observational constraints; however, the giant-cell velocities at the surface are larger than observed. The large-scale magnetic field, induced by the differential rotation and helical motions, peaks in the subadiabatic region below the convection zone; and, near the top of the convection zone, it is concentrated over the downdrafts of giant cells. A systematic drift in latitude of the magnetic field or a field reversal has not yet been seen.