Numerical simulations of planetary and stellar dynamos

The magnetic fields of planets and stars are sustained by dynamo action operating in planetary cores or atmospheres [1], or stellar convection zones. The process involves the generation of electric currents via convective motions of the electrically conducting fluid [3]. Despite the motions being spatially disorganised, dynamo action has sustained globally coherent magnetic fields in bodies throughout the Solar System (and beyond) for the majority of its lifetime. Typically, for terrestrial planets (e.g. Earth), including those whose dynamo has ceased to operate (e.g. Mars), and moons (e.g. Ganymede), an iron-rich fluid is located in a (outer) core region deep beneath the surface; for giant planets (e.g. Jupiter, Saturn) the conducting fluid arises by the formation of metallic hydrogen in the high-pressure atmospheres; for stars the electrically conducting plasma enables dynamo action. Despite these differences in fluid properties and location, the various magnetic fields produced have many common characteristics such as domination of dipolar field and dynamics existing on a wide range of length- and time-scales. How the dynamo process exactly operates and how the dynamics of the celestial bodies are affected are outstanding questions in planetary and stellar sciences. Research into the dynamics of dynamo regions will further aid our understanding of this problem and better our ability to predict future changes in planetary and stellar magnetic fields. Spherical dynamos are complicated problems where the spatial and temporal dynamics of convectively driven flows and electromagnetic induction must be studied together in a branch of physicsknown as magnetohydrodynamics. Progress is impeded because it is not possible to directly probe the region where dynamos operate [2]. We must therefore build models of the flow and magnetic field and test them by comparing their outputs with data from observations of naturally-occurring magnetic fields. The aim of the proposed research of this PhD project is to: 1) design new and build on existing analytical and numerical models of fluids and magnetic fields in planetary and stellainteriors; 2) perform numerical calculations and full simulations in representative parameter regimes; 3) study simulation data, perform diagnostics, and compare with analytical theory and observational data.References [1] E. Bullard and H. Gellman. Homogeneous dynamos and terrestrial magnetism. Phil. Trans. R.Soc. A, 247(928):213-278, 1954. [2] D. Gubbins and J. Bloxham. Geomagnetic field analysis. part iii. magnetic fields on the coremantle boundary. Geophys. J. R. Astr. Soc., 80:695-713, 1985. [3] H.K. Moffatt. Field Generation in Electrically Conducting Fluids. 1978.