Magnetohydrodynamics of Planetary and Solar Interiors

Many planets (for example, Ganymede, Mercury, the Earth, Jupiter and Saturn)and stars (for example, the Sun)possess intrinsic magnetic fields. The explanation for their existence and variation remains a great challenge to planetary scientists and astrophysicists. This article attempts to review some recent developments and difficulties in the study of magnetohydrodynamics for the electrically conducting fluid interiors of planets and stars. It is the fluid motions, usually driven by thermal convection, that generate and sustain magnetic fields through magnetohydrodynamic processes in planets and stars. In planets, the magnetohydrodynamic processes are strongly affected by the combined and inseparable effects of rotation, magnetic fields and spherical geometry. The key dynamics involves the interaction between the Coriolis and Lorentz forces. In the Sun, it is the solar tachocline, a thin shear flow layer located at the base of the convection envelope, that plays an essential role in the solar magnetohydrodynamic processes which produce the 11-year sunspot cycle. Some results of a new nonlinear three-dimensional solar dynamo model are also presented.