Core surface magnetic field evolution 2000-2010

Abstract

SUMMARY We present new dedicated core surface field models spanning the decade from 2000.0 to 2010.0. These models, called gufm‐sat, are based on CHAMP, Ørsted and SAC‐C satellite observations along with annual differences of processed observatory monthly means. A spatial parametrization of spherical harmonics up to degree and order 24 and a temporal parametrization of sixth‐order B‐splines with 0.25 yr knot spacing is employed. Models were constructed by minimizing an absolute deviation measure of misfit along with measures of spatial and temporal complexity at the core surface. We investigate traditional quadratic or maximum entropy regularization in space, and second or third time derivative regularization in time. Entropy regularization allows the construction of models with approximately constant spectral slope at the core surface, avoiding both the divergence characteristic of the crustal field and the unrealistic rapid decay typical of quadratic regularization at degrees above 12. We describe in detail aspects of the models that are relevant to core dynamics. Secular variation and secular acceleration are found to be of lower amplitude under the Pacific hemisphere where the core field is weaker. Rapid field evolution is observed under the eastern Indian Ocean associated with the growth and drift of an intense low latitude flux patch. We also find that the present axial dipole decay arises from a combination of subtle changes in the southern hemisphere field morphology.

Limitations in paleomagnetic data and modelling techniques and their impact on Holocene geomagnetic field models

“…Various choices of regularization norm in time and space have been explored by different authors (e.g. Bloxham & Jackson 1992;Jackson et al 2007;Gillet et al 2007;Korte & Holme 2010;Finlay et al 2012). In this study we used the Ohmic dissipation norm at the CMB (Gubbins & Bloxham 1985;Jackson et al 2000) for the spatial regularization:…”

“…Claerbout & Muir 1973;Scales et al 1988;Farquharson & Oldenburg 1998;Tarantola 2005). The L 1 norm measure of misfit has been successfully employed in geomagnetic field modelling of recent and historical data Lesur et al 2008;Finlay et al 2012) and regional archeomagnetic field models (Pavón-Carrasco et al 2009), but it has not previously been tested in the construction of global field models on millennial time scales.…”

Canonical Models of Geophysical and Astrophysical Flows: Turbulent Convection Experiments in Liquid Metals

“…For example, Earth's geomagnetic field is shown in Figure 1a [7]. The field is predominantly dipolar, with magnetic flux emerging from Earth's southern hemisphere and returning via the northern hemisphere.…”

“…[7]. The field is dominated by the axial dipole component, but also has rich higher order structure; (b) Secular variation of the 2005 radial CMB field, measured in units of µT /yr [7]. The radial field component can change by a few percent per year; (c) Schematic of Earth's interior with the silicate mantle overlying the iron-rich core, all rotating with angular velocity Ωẑ.…”

“…Generated by the motion of the electrically-conductive fluid, dynamo fields typically evolve on length and time scales related to the underlying flows [6]. [7]. The field is dominated by the axial dipole component, but also has rich higher order structure; (b) Secular variation of the 2005 radial CMB field, measured in units of µT /yr [7].…”