Geomagnetic field for 0–3 ka: 2. A new series of time‐varying global models

Abstract

[1] Steadily increasing numbers of archeomagnetic and paleomagnetic data for the Holocene have allowed development of temporally continuous global spherical harmonic models of the geomagnetic field extending present and historical global descriptions of magnetic field evolution. The current work uses various subsets of improved data compilations, details of which are given in a companion paper by Donadini et al. (2009), and minor modifications of standard modeling strategies (using temporally and spatially regularized inversion of the data and cubic spline parameterizations for temporal variations) to produce five models with enhanced spatial and temporal resolution for 0-3 ka. Spurious end effects present in earlier models are eliminated by enforcing large-scale agreement with the gufm1 historical model for 1650-1990 A.D. and by extending the model range to accommodate data older than 3 ka. Age errors are not considered as a contribution to data uncertainties but are included along with data uncertainties in an investigation of statistical uncertainty estimates for the models using parametric bootstrap resampling techniques. We find common features but also significant differences among the various models, indicating intrinsic uncertainties in global models based on the currently available Holocene data. Model CALS3k.3 based on all available archeomagnetic and sediment data, without a priori quality selection, currently constitutes the best global representation of the past field. The new models have slightly higher dipole moments than our previous models. Virtual axial dipole moments (VADMs) calculated directly from the data are in good agreement with all corresponding model predictions of VADMs. These are always higher than the spherical harmonic dipole moment, indicating the limitations of using VADMs as a measure of geomagnetic dipole moments.

Geomagnetic intensity variations for the past 8 kyr: New archaeointensity results from Eastern China

“…When comparing our data with the predictions of the variation of geomagnetic field at the center of China (35 • N, 105 • E) from the three global models, our new data fit well with the ARCH3k.1 model (Korte et al, 2009) but deviate significantly at certain periods from the CALS3k.4 and CALS10k.1b model (Fig. 6).…”

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

“…To mitigate this effect, we follow Korte et al (2009) and exclude the dipole terms (l = 1, m = 0, 1) from the spatial regularization and the summation over l starts at 2 instead of 1 in eq. (8).…”

“…assigned minimum uncertainties for sediment records of 3.5 • in inclination, 5.0 • in declination and 5 µT in intensity, based on the comparisons with the historical model gufm1 . For the more recent CALSxk models, minimum uncertainties for sediment directional data have been expressed in terms of an α 95 of 6 • Korte et al 2009). To take into account both data and age uncertainties, recent models CALS3k.4b and CALS10k.1b were obtained by averaging multiple models built with resampling of the data sets across distributions representing their estimated data and age uncertainties .…”

“…Several modelling methodologies have been attempted to assess the influences of these limitations. The CALSx series of global geomagnetic models (Korte & Constable 2003, 2005Korte et al 2009; covering the past 3, 7 and 10 kyr use spherical harmonic expansions and cubic spline temporal basis functions together with regularization methods to find the models with minimum complexity that fit the data to the desired accuracy. Licht et al (2013) presented three ensembles of low spherical harmonic degree field models (truncated at degree and order 5) that cover the past three millennia, built from archeomagnetic, volcanic and sedimentary data sets.…”

Directional changes of the geomagnetic field in West Africa: Insights from the metallurgical site of Korsimoro

“…The outcome of the reconstruction in terms of global models (e.g. Korte et al, 2009Licht et al, 2013), regional models (e.g. Pavón-Carrasco et al, 2009, or secular variation curves (e.g.…”