Comparison of terrestrial and solar dynamos

Abstract

The Earth's magnetic field has undergone temporal and spatial variations including polarity reversals. Paleomagnetic and historical magnetic field measurements suggest persistent distinct patterns of variation of the geomagnetic field taking place in different regions of the Earth. These patterns can be explained by core–mantle thermal interaction in which lateral variations in heat flux across the core–mantle boundary drive core flows at the top of the Earth's fluid core. The solar magnetic field has also undergone variation on widely separated scales. It is generally believed that the nearly 22-year sunspot cycle and its spatial symmetry with respect to the equator are a consequence of magnetohydrodynamic processes taking place in a highly differentially rotating layer between the convective and radiative regions of the Sun.For the Earth, the task of modelling an Earth-like and self-sustaining dynamo remains a major challenge because of the length scale disparities associated with the extremely small Ekman number of the Earth's fluid outer core. The scale disparities are not only the root of severe difficulties in modelling the geodynamo but they are also characteristic of the geodynamo dynamics. For the Sun, the solar tachocline offers an ideal location for the generation and storage of the Sun's strong azimuthal magnetic fields while the large-scale solar surface magnetic activity represents the rising and emerging of deep-seated, strong toroidal magnetic fields driven by magnetic buoyancy. However, a global solar dynamo model which dynamically incorporates the radiative core and is capable of reproducing a self-consistent and nearly solid-body rotating core with an overlying strongly radial shear layer remains a major challenge.Significant progress has been made towards understanding the complex dynamo processes in the Earth and Sun. We discuss the main differences and similarities between the geomagnetic and solar magnetic fields. We also discuss why different approaches have been employed in the numerical modelling of the geodynamo and solar dynamo and what are the major difficulties in simulating both dynamos. We employ two spherical non-linear dynamo models to illustrate how the observed geomagnetic or solar magnetic fields can be controlled by the effect of the radial shear layers which exist in the Sun's interior and might exist in the Earth's core.