A statistical analysis of magnetic fields from some geodynamo simulations

We present a statistical analysis of magnetic fields simulated by the Glatz maier-Roberts dynamically consistent dynamo model. For four simulations wit h distinct boundary conditions, means, standard deviations, and probability functions permit an evaluation based on existing statistical paleosecular variation (PSV) models. Although none closely fits the statistical PSV mode ls in all respects, some simulations display characteristics of the statist ical PSV models in individual tests. We also find that nonzonal field stati stics do not necessarily reflect heat flow conditions at the core-mantle bo undary. Multitaper estimates of power and coherence spectra allow analysis of time series of single, or groups of, spherical harmonic coefficients rep resenting the magnetic fields of the dynamo simulations outside the core. S liding window analyses of both power and coherence spectra from two of the simulations show that a 100 kyr averaging time is necessary to realize stat ionary statistics of their nondipole fields and that a length of 350 kyr is not long enough to full characterize their dipole fields. Spectral analysi s provides new insight into the behavior and interaction of the dominant co mponents of the simulated magnetic fields, the axial dipole and quadrupole. Although we find spectral similarities between several reversals, there is no evidence of signatures that can be conclusively associated with reversa ls or excursions. We test suggestions that during reversals there is increa sed coupling between groups of spherical harmonic components. Despite evide nce of coupling between antisymmetric and symmetric spherical harmonics in one simulation, we conclude that it is rare and not directly linked to reve rsals. In contrast to the reversal model of R. T. Merrill and P. L. McFadde n, we demonstrate that the geomagnetic power in the dipole part of the dyna mo simulations is either relatively constant or fluctuates synchronously wi th that of the nondipole part and that coupling between antisymmetric and s ymmetric components occurs when the geomagnetic power is high.