The ‘dynamo problem’ requires that the origin of the primarily dipole geomagnetic
field be found. The source of the geomagnetic field lies within the outer core of
the Earth, which contains a turbulent magnetofluid whose motion is described by the
equations of magnetohydrodynamics (MHD). A mathematical model can be based on the
approximate but essential features of the problem, i.e., a rotating spherical shell containing
an incompressible turbulent magnetofluid that is either ideal or real but maintained in
an equilibrium state. Galerkin methods use orthogonal function expansions to represent
dynamical fields, with each orthogonal function individually satisfying imposed boundary
conditions. These Galerkin methods transform the problem from a few partial differential
equations in physical space into a huge number of coupled, non-linear ordinary differential
equations in the phase space of expansion coefficients, creating a dynamical system. In
the ideal case, using Dirichlet boundary conditions, equilibrium statistical mechanics has
provided a solution to the problem. As has been presented elsewhere, the solution also
has relevance to the non-ideal case. Here, we examine and compare Galerkin methods
imposing Neumann or mixed boundary conditions, in addition to Dirichlet conditions.
Any of these Galerkin methods produce a dynamical system representing MHD turbulence
and the application of equilibrium statistical mechanics in the ideal case gives solutions
of the dynamo problem that differ only slightly in their individual sets of wavenumbers.
One set of boundary conditions, Neumann on the outer and Dirichlet on the inner surface,
might seem appropriate for modeling the outer core as it allows for a non-zero radial component
of the internal, turbulent magnetic field to emerge and form the geomagnetic field.
However, this does not provide the necessary transition of a turbulent MHD energy spectrum
to match that of the surface geomagnetic field. Instead, we conclude that the model
with Dirichlet conditions on both the outer and the inner surfaces is the most appropriate
because it provides for a correct transition of the magnetic field, through an electrically
conducting mantle, from the Earth’s outer core to its surface, solving the dynamo problem.
In addition, we consider how a Galerkin model velocity field can satisfy no-slip conditions
on solid boundaries and conclude that some slight, kinetically driven compressibility must
exist, and we show how this can be accomplished.
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