Theory of the geodynamo: 100 years of struggle and a new dawn.

Earth's magnetic field is generated by fluid motion in the outer core
by a process termed self-exciting dynamo action. In this process, electrically conducting fluid
flows through a magnetic field, inducing electrical currents that reinforce the original
magnetic field. The driving force for this is thought to be thermal convection.

This process can be simulated on the computer in a self-consistent way, albeit in a
parameter regime that is somewhat distant from planetary settings. In particular, the values of
viscosity used are too large, and the prospects for reducing these viscosities to more appropriate
values are remote. Despite this, the approach has met with great success and has demonstrated that
magnetic fields can be generated in this way. Many features are quite Earth-like,
most likely because the magnetic Reynolds number (the ratio of magnetic induction to magnetic 
diffusion) is in the correct regime. We will contrast conventional models with a different approach in 
which both inertia and viscosity are omitted from the equations at the outset. This approach,
whilst in its infancy, holds the promise of providing complementary models of planetary magnetic field generation.