UC Santa Cruz

Many celestial bodies possess magnetic fields generated somehow by dynamo action within the interior of the object. These magnetic fields can be dynamically significant. For example, sunspots in magnetically-active regions of the sun are related to the flares and coronal mass ejections which create the space weather that influences life on Earth. Furthermore, the earthly protection from such events is created by the Earth's interior geodynamo. Understanding such dynamics is in general a complex highly nonlinear problem and we frequently turn to numerical simulations. However, for many of these circumstances, even modern supercomputers lack the power to model the extreme parameter regimes of the true objects. For that reason, we often resort to simpler simulations as experimental laboratories to explore ideas that might be relevant. In this thesis, we adopt this approach and use direct numerical simulations to study some unusual ideas on aspects of MHD phenomena relating to the solar, geo, and lunar dynamos.

The first of these novel studies examines the operation of essentially nonlinear dynamos (ENDs), where nonlinear effects are dominant from the start (as opposed to becoming important after a linear epoch). Here the model design is inspired by the solar tachocline where there is a strong toroidal shear, but this END study has additional relevance to the subcritical nature of the geodynamo. In a second study, we employ another novel theory, that of stoked nondynamos to provide an alternative scenario for some unexplained observations of the lunar magnetic field. Here, we examine a non-closed system that combines a lunar core dynamo with a surrounding nondynamo basal magma ocean as a plausible mechanism to explain unexpectedly high paleointensities found in lunar surface rocks. In the final study of this thesis, we return to the Sun to investigate the origin of the solar hemispheric helicity rules (SHHR) via three dimensional simulations of twisted magnetic flux tubes in the presence of rotating convection. This study examines the efficacy of a highly-simplified theoretical mechanism, the $\Sigma$-effect, via fully resolved three dimensional MHD simulations.