○ September 18th (Wed)
Time 
14:4515:45 
Speaker 
Peter A. Davidson
Department of Engineering, University of Cambridge, UK 
Title 
Scaling laws for planetary dynamos 
Abstract 
We propose new scaling laws for the properties of planetary dynamos. In particular, the Rossby number, the magnetic Reynolds number, the ratio of magnetic to kinetic energy, the Ohmic dissipation timescale and the characteristic aspect ratio of the columnar convection cells are all predicted to be powerlaw functions of two observable quantities: the magnetic dipole moment and the planetary rotation rate. The resulting scaling laws constitute a somewhat modified version of those proposed by Christensen. The main difference is that, in view of the small value of the Rossby number in planetary cores, we insist that the nonlinear inertial term, u・∇u, is negligible. This changes the exponents in the powerlaws which relate the various properties of the fluid dynamo to the planetary dipole moment and rotation rate. These scaling laws are consistent with the available numerical evidence.





