  It has been suggested in Hoyng (2009) that dynamo action can be analysed by
expansion of the magnetic field into dynamo modes and statistical evaluation of
the mode coefficients. We here validate this method by analysing a numerical
geodynamo model and comparing the numerically derived mean mode coefficients
with the theoretical predictions. The model belongs to the class of
kinematically stable dynamos with a dominating axisymmetric, antisymmetric with
respect to the equator and non-periodic fundamental dynamo mode. The analysis
requires a number of steps: the computation of the so-called dynamo
coefficients, the derivation of the temporally and azimuthally averaged dynamo
eigenmodes and the decomposition of the magnetic field of the numerical
geodynamo model into the eigenmodes. For the determination of the theoretical
mode excitation levels the turbulent velocity field needs to be projected on
the dynamo eigenmodes. We compare the theoretically and numerically derived
mean mode coefficients and find reasonably good agreement for most of the
modes. Some deviation might be attributable to the approximation involved in
the theory. Since the dynamo eigenmodes are not self-adjoint a spectral
interpretation of the eigenmodes is not possible.
