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Panovska, S, Korte M, Finlay CC, Constable CG.  2015.  Limitations in paleomagnetic data and modelling techniques and their impact on Holocene geomagnetic field models. Geophysical Journal International. 202:402-418.   10.1093/gji/ggv137   AbstractWebsite

Characterization of geomagnetic field behaviour on timescales of centuries to millennia is necessary to understand the mechanisms that sustain the geodynamo and drive its evolution. As Holocene paleomagnetic and archeomagnetic data have become more abundant, strategies for regularized inversion of modern field data have been adapted to produce numerous time-varying global field models. We evaluate the effectiveness of several approaches to inversion and data handling, by assessing both global and regional properties of the resulting models. Global Holocene field models cannot resolve Southern hemisphere regional field variations without the use of sediments. A standard data set is used to construct multiple models using two different strategies for relative paleointensity calibration and declination orientation and a selection of starting models in the inversion procedure. When data uncertainties are considered, the results are similar overall regardless of whether we use iterative calibration and reorientation, or co-estimation of the calibration and orientation parameters as part of the inversion procedure. In each case the quality of the starting model used for initial relative paleointensity calibration and declination orientation is crucial and must be based on the best absolute information available. Without adequate initial calibration the morphology of dipole moment variations can be recovered but its absolute value will be correlated with the initial intensity calibrations, an effect that might be mitigated by ensuring an appropriate fit to enough high quality absolute intensity data with low uncertainties. The declination reorientation mainly impacts regional field structure and in the presence of non-zonal fields will result in a non-zero local average. The importance of declination orientation is highlighted by inconsistencies in the West Pacific and Australian sediment records in CALS10k.1b model. Great care must also be taken to assess uncertainties associated with both paleomagnetic and age data and to evaluate the effects of poor data distribution. New consistently allocated uncertainty estimates for sediment paleomagnetic records highlight the importance of adequate uncertainties in the inversion process, as they determine the relative weighting among the data and overall normalized misfit levels which in turn influence the complexity of the inferred field models. Residual distributions suggest that the most appropriate misfit measure is the L-1 norm (minimum absolute deviation) rather than L-2 (least squares), but this seems to have relatively minor impact on the overall results. For future Holocene field modelling we see a need for comprehensive methods to assess uncertainty in individual archeomagnetic data so that these data or models derived from them can be used for reliable initial relative paleointensity calibration and declination orientation in sediments. More work will be needed to assess whether co-estimation or an iterative approach to inversion is more efficient overall. This would be facilitated by realistic and globally consistent data and age uncertainties from the paleomagnetic community.

Smith-Boughner, LT, Constable CG.  2012.  Spectral estimation for geophysical time-series with inconvenient gaps. Geophysical Journal International. 190:1404-1422.   10.1111/j.1365-246X.2012.05594.x   AbstractWebsite

The power of spectral estimation as a tool for studying geophysical processes is often limited by short records or breaks in available time-series. Direct spectral estimation using multitaper techniques designed to reduce variance and minimize leakage can help alleviate the first problem. For records with gaps, systematic interpolation or averaging of multitaper spectra derived from record fragments may prove adequate in some cases, but can be cumbersome to implement. Alternatively, multitapers can be modified for use in direct spectral estimation with intermittently sampled data. However, their performance has not been adequately studied. We investigate reliability and resolution of techniques that adapt prolate and minimum bias (MB) multitapers to accommodate the longest breaks in sampling, comparing the tapering functions (referred to as PRG or MBG tapers) with the standard prolate and MB tapers used for complete data series, and with the section-averaging approach. Using a synthetic data set, we test both jackknife and bootstrap methods to calculate confidence intervals for PRG and MBG multitaper spectral estimates and find the jackknife is both more accurate and faster to compute. To implement these techniques for a variety of data sets, we provide an algorithm that allows the user to balance judicious interpolation against the use of suitably adapted tapers, providing empirical measures of both bias and frequency resolution for candidate sets of tapers. These techniques are tested on diverse geophysical data sets: a record of change in the length of day, a model of the external dipole part of the geomagnetic field produced by the magnetospheric ring current, and a 12 Myr long irregularly sampled relative geomagnetic palaeointensity record with pernicious gaps. We conclude that both PRG and MBG tapers generally perform as well as, or better than, an optimized form of the commonly used section averaging approach. The greatest improvements seem to occur when the gap structure creates data segments of very unequal lengths. Ease of computation and more robust behaviour can make MBG tapers a better choice than PRG except when very fine-scale frequency resolution is required. These techniques could readily be applied for cross-spectral and transfer function estimation and are a useful addition to the geophysical toolbox.

Ribaudo, JT, Constable CG, Parker RL.  2012.  Scripted finite element tools for global electromagnetic induction studies. Geophysical Journal International. 188:435-446.   10.1111/j.1365-246X.2011.05255.x   AbstractWebsite

Numerical solution of global geomagnetic induction problems in two and three spatial dimensions can be conducted with commercially available, general-purpose, scripted, finite-element software. We show that FlexPDE is capable of solving a variety of global geomagnetic induction problems. The models treated can include arbitrary electrical conductivity of the core and mantle, arbitrary spatial structure and time behaviour of the primary magnetic field. A thin surface layer of laterally heterogeneous conductivity, representing the oceans and crust, may be represented by a boundary condition at the Earthspace interface. We describe a numerical test, or validation, of the program by comparing its output to analytic and semi-analytic solutions for several electromagnetic induction problems: (1) concentric spherical shells representing a layered Earth in a time-varying, uniform, external magnetic field, (2) eccentrically nested conductive spheres in the same field and (3) homogeneous spheres or cylinders, initially at rest, then rotating at a steady rate in a constant, uniform, external field. Calculations are performed in both the time and frequency domains, and in both 2-D and 3-D computational meshes, with adaptive mesh refinement. Root-mean-square accuracies of better than 1 per cent are achieved in all cases. A unique advantage of our technique is the ability to model Earth rotation in both the time and the frequency domain, which is especially useful for simulating satellite data.

Ziegler, LB, Constable CG, Johnson CL, Tauxe L.  2011.  PADM2M: a penalized maximum likelihood model of the 0-2 Ma palaeomagnetic axial dipole moment. Geophysical Journal International. 184:1069-1089.   10.1111/j.1365-246X.2010.04905.x   AbstractWebsite

P>We present a new time-varying model for palaeomagnetic axial dipole moment (PADM) for the past 2 Myr and compare it with earlier virtual axial dipole moment (VADM) reconstructions which have been based on stacking and averaging scaled relative palaeointensity records. The PADM is derived from both absolute and relative palaeointensity data and constructed using a new penalized maximum likelihood (PML) approach to recover a cubic B-spline representation of axial-dipole field variations on million year timescales. The PML method is explicitly intended to reduce bias in estimating the true axial dipole moment that arises in average VADM reconstructions. We apply the PML method to a set of 96 032 published data (1800 palaeointensities from igneous rocks, 3300 archaeointensities and 86 relative palaeointensity time-series of variable lengths and resolutions). Two models are discussed: PADM2Mp is a trial model based on a subset of the nine longest available sedimentary records; PADM2M uses a comprehensive data set (76 records, 81 446 data; 10 records were eliminated) and is our preferred model. PADM2M has a lower mean than existing VADM reconstructions but shows similarities in long-period variability. Some differences in timing, amplitude and resolution of certain features can be attributed to variations in age assignments. Others result from our more comprehensive data set and a reduction in bias attributable to PML modelling. PADM2M has an average axial dipole moment over 0-2 Ma of 5.3 x 1022 Am2 with a standard deviation of 1.5 x 1022 Am2. The Brunhes chron average (6.2 x 1022 Am2) is higher than for earlier epochs of Matuyama (4.8 x 1022 Am2), as seen in some previous studies. The power spectrum for our model agrees with previous estimates of the global palaeomagnetic power spectrum for frequencies up to about 102 Myr-1. We see no distinctive evidence in the power spectrum for orbital forcing of geodynamo behaviour.

Johnson, CL, Wijbrans JR, Constable CG, Gee J, Staudigel H, Tauxe L, Forjaz VH, Salgueiro M.  1998.  Ar-40/Ar-39 ages and paleomagnetism of Sao Miguel lavas, Azores. Earth and Planetary Science Letters. 160:637-649.   10.1016/s0012-821x(98)00117-4   AbstractWebsite

We present new Ar-40/Ar-39 ages and paleomagnetic data for Sao Miguel island, Azores. Paleomagnetic samples were obtained for 34 flows and one dike; successful mean paleomagnetic directions were obtained for 28 of these 35 sites. Ar-40/Ar-39 age determinations on 12 flows from the Nordeste complex were attempted successfully: ages obtained are between 0.78 Ma and 0.88 Ma, in contrast to published K-Ar ages of 1 Ma to 4 Ma. Our radiometric ages are consistent with the reverse polarity paleomagnetic field directions, and indicate that the entire exposed part of the Nordeste complex is of a late Matuyama age. The duration of volcanism across Sao Miguel is significantly less than previously believed, which has important implications for regional melt generation processes, and temporal sampling of the geomagnetic field. Observed stable isotope and trace element trends across the island can be explained, at least in part, by communication between different magma source regions at depth. The Ar-40/Ar-39 ages indicate that our normal polarity paleomagnetic data sample at least 0.1 Myr (0-0.1 Ma) and up to 0.78 Myr (0-0.78 Ma) of paleosecular variation and our reverse polarity data sample approximately 0.1 Myr (0.78-0.88 Ma) of paleosecular variation. Our results demonstrate that precise radiometric dating of numerous flows sampled is essential to accurate inferences of long-term geomagnetic field behavior. Negative inclination anomalies are observed for both the normal and reverse polarity time-averaged field. Within the data uncertainties, normal and reverse polarity field directions are antipodal, but the reverse polarity field shows a significant deviation from a geocentric axial dipole direction. (C) 1998 Elsevier Science B.V. All rights reserved.