Recent advances in planetary sciences

The progress in handling liquid metal flows in large containers has opened opportunities for the realisation of homogeneous dynamos in the laboratory. Combined with increased computer capacity that has permitted numerical experiments of convention-driven dynamos, these developments will lead to a deeper understanding of the origin of fields in planets and in stars.

Energy

The origin of the Earth's magnetic field, as most cosmic bodies' magnetic fields, is attributed to the motion of electrically conducting fluids. Whereas the underlying theory of homogeneous dynamos has been widely elaborated over the last decades, only recently has an experimental verification of magnetic field self-excitation in conducting fluids been pursued.

The experiments conducted at the Riga dynamo facility in November 1999 had shown that a liquid metal flow spiralling in a cylindrical container can generate a slowly growing magnetic field. The European Commission funded MAGDYN project's main objective was to upgrade and further exploit this laboratory facility through new series of dynamo experiments.

The experiments relied on numerical simulations to calculate the fluid flow and the induced magnetic field, both in the design phase and for data analysis. Two equation eddy-viscosity and full Reynolds-stress numerical models which solve the MHD equations in the experimental geometry were developed by project partners at the Delft University of Technology.

Despite all the approximations involved in the computational calculations, the comparison of the experimental results with the simulations predictions of central quantities showed a satisfactory agreement. A next-generation large-scale dynamo experiment that would bring most valuable insight on the energetic and magnetostrophic balance of planetary dynamos is being designed on the basis of these results. While the kinematic dynamo effect and the role of high levels of turbulence on the self-excitation conditions have already been investigated in the Riga experiment, there are still open questions that need to be addressed.

The figure caption: Results of the numerical simulations of the Riga dynamo: Reynolds number is 3.5x10^6, Reynolds magnetic number is 18. Figure illustrates spatial distributions of growing magnetic field visualised by combination of iso-surfaces of axial magnetic field (red - positive, blue - negative) with magnetic flux lines grey tubes).