We have used a comprehensive data compilation and recently refined modelling strategies to produce CALS10k.1b, the first time-varying spherical harmonic geomagnetic field model spanning 10 ky. The model is an average obtained from bootstrap sampling to take account of uncertainties in magnetic components and ages in the data (and hence has version number 1b instead of 1). This model shows less spatial and temporal resolution than earlier versions for 0 - 3 ka, and particularly aims to provide a robust representation of the large-scale field at the core-mantle boundary (CMB).

Reference:  

• Korte, M., C. Constable, F. Donadini and R. Holme (2011): Reconstructing the Holocene Geomagnetic Field. Earth Planet Sci. Lett., 312, 497-505

Downloads:

CALS10k_1b.zip, 750 kB, Dec 2011, Model file CALS10k.1b with Fortran source code to obtain time series of field predictions at the Earth's surface with uncertainty estimates (CALS10kfield.f) and to obtain the GAUSS coefficients for one point in time (CALS10kcoefs.f).

CALS10k_1b_anim.zip, 33MB, Dec. 2011, Animations of radial field component (Br) at the core-mantle boundary and of field intensity at the Earth's surface.

DM_CALS10k_1b.zip, 80kB, Dec. 2011, Plain text file with dipole moment estimate from CALS10k.1b.

Global geomagnetic field reconstructions on millennial time scales can be based on comprehensive paleomagnetic data compilations but, especially for older data, these still suffer from limitations in data quality and age controls as well as poor temporal and spatial coverage. Here we present updated global models for the time interval 0 - 3 ka where additions to the data basis mainly impact the South-East Asian, Alaskan, and Siberian regions. Bootstrap experiments to generate uncertainty estimates for the model take account of uncertainties in both age and magnetic elements and additionally assess the impact of  sampling in both time and space. Based on averaged results from bootstrap experiments, taking account of data and age uncertainties, we distinguish more conservative model estimates CALS3k.nb representing robust field structure at the core–mantle boundary from relatively high resolution models CALS3k.n for model versions n = 3 and 4. We presently consider CALS3k.4 the best high resolution model and recommend the more conservative lower resolution version for studies of field evolution at the CMB.

Reference:  

• Korte, M. and C. Constable (2011): Improving geomagnetic field reconstructions for 0 - 3ka. Phys. Earth Planet. Int., 188, 247-259

Downloads:

cals3k-4.zip, 2.7MB, Nov. 2011, Four ASCII files with coefficients: CALS3k.4, CALS3k.4b, CALS3k.3, CALS3k.3b. Fortran source code fieldpred3k.f to produces time series of declination, inclination and intensity for any location on the Earth's surface from any of the four models, and fielduncert3k.f to produces time series with uncertainty estimates based on the bootstrap from models CALS3k.4b or CALS3k.3b.

CALS3k-4anim.zip, 3.9MB, Nov. 2011, Animations of radial field component (Br) at the core-mantle-boundary as predicted by the four models and of the radial field differences between model versions 3 and 4.

This series of 5 main field models spanning the past 3kyrs was created to obtain a better understanding of the possibilities and limitations offered by different data types to reconstruct the global geomagnetic main field of the Holocene. Archeomagnetic data are generally considered to offer smaller uncertainties and particularly better age control than sediment records. Moreover, sediment records can at most provide information on relative intensity variation, but not on absolute paleointensity. On the other hand, archeomagnetic data are sparse prior to 1000 BC and from the southern hemisphere in general. Longer term global field reconstructions necessarily have to rely on sedimentary data, and even 3kyr models based only on archeomagnetic data are biased by the hemispherically assymmetric data distribution. Details are given in the references below. We conclude that models based on all available data, both archeomagnetic and sediments like in CALS3k.3, at present provide the most reliable reconstruction of the past geomagentic field evolution.

The five models are:

• CALS3k.3 based on all available data and with uncertainty estimates based on a bootstrap method.
• CALS3k_cst.1 based on all data types but with prior quality selection.
• ARCH3k.1 based only on archeomagnetic data and with uncertainty estimates based on a bootstrap method.
• ARCH3k_cst.1 based on archeomagnetic data with prior quality selection.
• SED3k.1 based only on sediment records where relative intensity has been scaled by the archeomagnetic model and with uncertainty estimates based on a bootstrap method.

References:

• Korte, M., F. Donadini and C. Constable (2009): Geomagnetic Field for 0-3ka: 2. A new Series of Time-varying Models. Geochem. Geophys. Geosys., 10, Q06008, doi:10.1029/2008GC002297
• Donadini, F., M. Korte and C. Constable (2009): Geomagnetic Field for 0-3ka: 1. New Data Sets for Global Modeling. Geochem. Geophys. Geosys., 10, Q06007, doi:10.1029/2008GC002295

Downloads:

CALS3k_3.zip, 1.5MB, May 2010, CALS3k_3 Model coefficients with uncertainty estimates and Fortran code for evaluation.

ARCH3k_1.zip, 30kB, May 2010, ARCH3k_1 Model coefficients with uncertainty estimates and Fortran code for evaluation.

SED3k_1.zip, 6.4MB, May 2010, SED3k_1 Model coefficients with uncertainty estimates and Fortran code for evaluation.

CALS3k_cst_1.zip, 6.5MB, May 2010, CALS3k_cst_1 Model coefficients and Fortran code for evaluation.

ARCH3k_cst_1.zip, 1.4MB, May 2010, ARCH3k_cst_1 Model coefficients and Fortran code for evaluation.

Dipole.zip, 700kB, May 2010, Dipole Text files with dipole moments (with uncertainties) for all 5 models.