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Avery, MS, Gee JS, Constable CG.  2017.  Asymmetry in growth and decay of the geomagnetic dipole revealed in seafloor magnetization. Earth and Planetary Science Letters. 467:79-88.   10.1016/j.epsl.2017.03.020   AbstractWebsite

Geomagnetic intensity fluctuations provide important constraints on time-scales associated with dynamical processes in the outer core. PADM2M is a reconstructed time series of the 0-2 Ma axial dipole moment (ADM). After smoothing to reject high frequency variations PADM2M's average growth rate is larger than its decay rate. The observed asymmetry in rates of change is compatible with longer term diffusive decay of the ADM balanced by advective growth on shorter time scales, and provides a potentially useful diagnostic for evaluating numerical geodynamo simulations. We re-analyze the PADM2M record using improved low-pass filtering to identify asymmetry and quantify its uncertainty via bootstrap methods before applying the new methodology to other kinds of records. Asymmetry in distribution of axial dipole moment derivatives is quantified using the geomagnetic skewness coefficient, sg. A positive value indicates the distribution has a longer positive tail and the average growth rate is greater than the average decay rate. The original asymmetry noted by Ziegler and Constable (2011) is significant and does not depend on the specifics of the analysis. A long-term record of geomagnetic intensity should also be preserved in the thermoremanent magnetization of oceanic crust recovered by inversion of stacked profiles of marine magnetic anomalies. These provide an independent means of verifying the asymmetry seen in PADM2M. We examine three near bottom surveys: a 0 to 780 ka record from the East Pacific Rise at 19 degrees S, a 0 to 5.2 Ma record from the Pacific Antarctic Ridge at 51 degrees S, and a chron C4Ar-C5r (9.3-11.2 Ma) record from the NE Pacific. All three records show an asymmetry similar in sense to PADM2M with geomagnetic skewness coefficients, s(g) > 0. Results from PADM2M and C4Ar-C5r are most robust, reflecting the higher quality of these geomagnetic records. Our results confirm that marine magnetic anomalies can carry a record of the asymmetric geomagnetic field behavior first found for 0-2 Ma in PADM2M, and show that it was also present during the earlier time interval from 9.3-11.2 Ma. (C) 2017 The Authors. Published by Elsevier B.V.

Constable, C, Korte M, Panovska S.  2016.  Persistent high paleosecular variation activity in southern hemisphere for at least 10,000 years. Earth and Planetary Science Letters. 453:78-86.   10.1016/j.epsl.2016.08.015   AbstractWebsite

Direct observations of the geomagnetic field show that secular variation is strong in the Atlantic hemisphere, and comparatively reduced in the Pacific region. The dipole has been decaying since at least 1840 AD, driven by growth and migration of reverse flux patches in the southern hemisphere. We investigate whether anything like this modern pattern of geomagnetic secular variation persists and can be detected in global paleomagnetic field models. Synthesis of results from two new time-varying spherical harmonic models shows that geographically distinct geomagnetic secular variation extends to at least 10000 BP. The models use the same database but differ in methodology, leading to some regional differences in results. Consistent large-scale surface features include strong average fields in the northern hemisphere and weaker fields with greater overall variability in the south. Longitudinal structure is present, with weaker average fields in the western Pacific than in the east, and prominent negative inclination anomalies extending beneath Indonesia, across Africa and to Brazil, but weaker anomalies in the central Pacific. Marginally positive inclination anomalies occur west of the Americas. Paleosecular variation activity peaks at high southern latitudes, and there is a pattern of reduced activity at equatorial and mid-latitudes beneath the Pacific. Although the dipole has exhibited both growth and decay over the interval 0-10 000 BP, our results show that geomagnetic paleosecular variation is preferentially focused in similar geographic regions to secular variation seen in the modern field. (C) 2016 The Authors. Published by Elsevier B.V.

Smith-Boughner, LT, Ziegler LB, Constable CG.  2011.  Changing spectrum of geomagnetic intensity variations in a fragmented 12 My sediment record from the Oligocene. Physics of the Earth and Planetary Interiors. 188:260-269.   10.1016/j.pepi.2011.07.011   AbstractWebsite

Time series of relative geomagnetic paleointensity variations derived from marine sediments can be calibrated using absolute data derived from igneous materials. The resulting records may be suitable for spectral analysis of geomagnetic dipole variations. This work re-evaluates the 12 My (22.74-34.77 Ma) sediment record from Deep Sea Drilling Project Leg 73, Site 522, that is a key data set for determining the paleomagnetic power spectrum in the frequency range 1-100 My(-1). The 12 My record is marred by uneven sampling, with the interval between samples ranging from 1 to 640 ky, and contains several gaps that are considered too long to interpolate. The relative intensity data are calibrated using 129 globally distributed absolute paleointensity data from the same time interval. The power spectrum of the resulting time series is estimated using direct multi-taper spectral estimation with prolate data tapers adapted to deal with missing sections in the time series. The longest record available for analysis is thereby extended from 5.3 to 12 My. The new paleomagnetic power spectrum confirms the presence of a broad spectral peak at around 8 My(-1) for the early Oligocene and uncovers a peak around 2.5 My(-1) in the late Oligocene. Both peaks may be linked to tiny wiggles in marine magnetic anomalies. The new analysis unambiguously verifies that there is lower overall power in the younger part of the record, where the reversal process appears to dominate the power spectrum of the paleosecular variation. A comparison of the late Oligocene spectrum with that of PADM2M, a model of paleomagnetic axial dipole variations for 0-2 Ma, reveals some broad similarities; both time periods have similar power levels and a reversal rate of 4 My(-1). During the early Oligocene the reversal rate is about a factor of two lower, the field strength is higher, and the secular variation is stronger, suggesting that a strong magnetic field inhibits reversals but produces more variability in field strength. (C) 2011 Elsevier B.V. All rights reserved.

Hulot, G, Finlay CC, Constable CG, Olsen N, Mandea M.  2010.  The Magnetic Field of Planet Earth. Space Science Reviews. 152:159-222.   10.1007/s11214-010-9644-0   AbstractWebsite

The magnetic field of the Earth is by far the best documented magnetic field of all known planets. Considerable progress has been made in our understanding of its characteristics and properties, thanks to the convergence of many different approaches and to the remarkable fact that surface rocks have quietly recorded much of its history. The usefulness of magnetic field charts for navigation and the dedication of a few individuals have also led to the patient construction of some of the longest series of quantitative observations in the history of science. More recently even more systematic observations have been made possible from space, leading to the possibility of observing the Earth's magnetic field in much more details than was previously possible. The progressive increase in computer power was also crucial, leading to advanced ways of handling and analyzing this considerable corpus of data. This possibility, together with the recent development of numerical simulations, has led to the development of a very active field in Earth science. In this paper, we make an attempt to provide an overview of where the scientific community currently stands in terms of observing, interpreting and understanding the past and present behavior of the so-called main magnetic field produced within the Earth's core. The various types of data are introduced and their specific properties explained. The way those data can be used to derive the time evolution of the core field, when this is possible, or statistical information, when no other option is available, is next described. Special care is taken to explain how information derived from each type of data can be patched together into a consistent description of how the core field has been behaving in the past. Interpretations of this behavior, from the shortest (1 yr) to the longest (virtually the age of the Earth) time scales are finally reviewed, underlining the respective roles of the magnetohydodynamics at work in the core, and of the slow dynamic evolution of the planet as a whole.

Genevey, A, Gallet Y, Constable CG, Korte M, Hulot G.  2008.  ArcheoInt: An upgraded compilation of geomagnetic field intensity data for the past ten millennia and its application to the recovery of the past dipole moment. Geochemistry Geophysics Geosystems. 9   10.1029/2007gc001881   AbstractWebsite

This paper presents a compilation of intensity data covering the past 10 millennia (ArcheoInt). This compilation, which upgrades the one of Korte et al. (2005), contains 3648 data and incorporates additional intensity and directional data sets. A large majority of these data (similar to 87%) were acquired on archeological artifacts, and the remaining similar to 13% correspond to data obtained from volcanic products. The present compilation also includes important metadata for evaluating the intensity data quality and providing a foundation to guide improved selection criteria. We show that similar to 50% of the data set fulfill reasonable reliability standards which take into account the anisotropic nature of most studied objects (potsherds), the stability of the magnetization, and the data dispersion. The temporal and geographical distributions of this sub-data set are similar to those of the main data set, with similar to 72% of the data dated from the past three millennia and similar to 76% obtained from western Eurasia. Approximately half of the selected intensity data are associated with at least an inclination value. To constrain the axial and full dipole evolution over the past three millennia requires that we avoid any overrepresentation of the western Eurasian data. We introduce a first-order regional weighting scheme based on the definition of eight widely distributed regions of 30 degrees width within which the selected data are numerous enough. The regional curves of virtual axial dipole moments (VADM) and of mixed VADM-virtual dipole moments (VDM) averaged over sliding windows of 200 years and 500 years testify for strong contributions from either equatorial dipole or nondipole components. The computation of global VADM and mixed VADM/VDM variation curves, assuming an equal weight for each region, yields a dipole evolution marked by a distinct minimum around 0 B.C./A.D. followed by a maximum around the third-fourth century A. D. A second minimum is present around the eighth century A. D. This variation pattern is compatible with the one deduced from earlier, more sophisticated analysis based on the inversion of both intensity and directional data. In particular, there is a good agreement among all VADMs and dipole moment estimates over the historical period, which further strengthens the validity of our weighting scheme.

Johnson, CL, Constable CG.  1997.  The time-averaged geomagnetic field: global and regional biases for 0-5 Ma. Geophysical Journal International. 131:643-+.   10.1111/j.1365-246X.1997.tb06604.x   AbstractWebsite

Palaeodirectional data from lava flows and marine sediments provide information about the long-term structure and variability in the geomagnetic held. We present a detailed analysis of the internal consistency and reliability of global compilations of sediment and lava-flow data. Time-averaged field models are constructed for normal and reverse polarity periods for the past 5 Ma, using the combined data sets. Non-zonal models are required to satisfy the lava-flow data, but not those from sediments alone. This is in part because the sediment data are much noisier than those from lavas, but is also a consequence of the site distributions and the way that inclination data sample the geomagnetic field generated in the Earth's core. Different average held configurations for normal and reverse polarity periods are consistent with the palaeomagnetic directions; however, the differences are insignificant relative to the uncertainty in the average field models. Thus previous inferences of non-antipodal normal and reverse polarity field geometries will need to be re-examined using recently collected high-quality palaeomagnetic data. Our new models indicate that current global sediment and lava-flow data sets combined do not permit the unambiguous detection of northern hemisphere flux lobes in the 0-5 Ma time-averaged field, highlighting the need for the collection of additional high-latitude palaeomagnetic data. Anomalous time-averaged held structure is seen in the Pacific hemisphere centred just south of Hawaii. The location of the anomaly coincides with heterogeneities in the lower mantle inferred from seismological data. The seismic observations can be partly explained by lateral temperature variations; however, they also suggest the presence of lateral compositional variations and/or the presence of partial melt. The role of such heterogeneities in influencing the geomagnetic held observed at the Earth's surface remains an unresolved issue, requiring higher-resolution time-averaged geomagnetic field models, along with the integration of future results from seismology, mineral physics and numerical simulations.

Johnson, CL, Constable CG.  1995.  The Time-Averaged Geomagnetic-Field As Recorded By Lava Flows Over The Past 5 Million-Years. Geophysical Journal International. 122:489-519.   10.1111/j.1365-246X.1995.tb07010.x   AbstractWebsite

A recently compiled lava flow data base spanning the last 5 million years is used to investigate properties of the time-averaged geomagnetic field. More than 90 per cent of the power in the palaeofield can be accounted for by a geocentric axial dipole; however, there are significant second-order structures in the held. Declination and inclination anomalies for the new data base indicate that the main second-order signal is the 'far-sided' effect, and there is also evidence for non-zonal structure. VGP (virtual geomagnetic pole) latitude distributions indicate that, over the last 5 million years, normal and reverse polarity morphologies are different, and that any changes in the normal polarity field morphology are undetectable, given the present data distribution. Regularized non-linear inversions of the palaeomagnetic directions support all these observations. We test the hypothesis that zonal models for the time-averaged field are adequate to describe the data and find that they are not. Non-zonal models are needed to fit the data to within the required tolerance level. Normal and reverse polarity held models obtained are significantly different. Field models obtained for the Brunhes epoch data alone are much smoother than those obtained from combining an the normal polarity data; simulations indicate that these differences can be explained by the less extensive data distribution for the Brunhes epoch. The field model for all of the normal polarity data (LN1) contains features observed in the historical field maps, although the details differ. LN1 suggests that, although the two northern hemisphere flux lobes observed in the historical field are stationary to a first-order approximation, they do show changes in position and amplitude. A. third, less pronounced flux lobe is observed in LN1 over central Europe. The lack of structure ih the southern hemisphere is due in part to the paucity of data. Jackknife estimates of the field models for different subsets of the data suggest that a few sites contribute significant structure to the final field models. More conservative estimates of the time-averaged field morphology are obtained by removing these sites.