We present a new model of the magnetic › eld at the core{mantle boundary for the interval 1590{1990. The model, called gufm1, is based on a massive new compilation of historical observations of the magnetic › eld. The greater part of the new dataset originates from unpublished observations taken by mariners engaged in merchant and naval shipping. Considerable attention is given to both correction of data for possible mislocation (originating from poor knowledge of longitude) and to proper allocation of error in the data. We adopt a stochastic model for uncorrected positional errors that properly accounts for the nature of the noise process based on a Brownian motion model. The variability of navigational errors as a function of the duration of the voyages that we have analysed is consistent with this model. For the period before 1800, more than 83000 individual observations of magnetic declination were recorded at more than 64000 locations; more than 8000 new observations are for the 17th century alone. The time-dependent › eld model that we construct from the dataset is parametrized spatially in terms of spherical harmonics and temporally in B-splines, using a total of 36512 parameters. The model has improved the resolution of the core › eld, and represents the longest continuous model of the › eld available. However, full exploitation of the database may demand a new modelling methodology.

https://www.everyday-feng-shui.de/feng-shui-news/wp-content/uploads/2012/06/2000-Jackson-957-90.pdf

Four centuries of geomagnetic secular variation from historical records

Recent seismological work has revealed new structures in the boundary layer between the Earth's core and mantle that are altering and expanding perspectives of the role this region plays in both core and mantle dynamics. Clear challenges for future research in seismological, experimental, theoretical and computational geophysics have emerged, holding the key to understanding both this dynamic system and geological phenomena observed at the Earth's surface.

The core–mantle boundary layer and deep Earth dynamics

The 75 year history of the American Geophysical Union has accompanied great advances in our understanding of the physics and chemistry of the transition zone between the Earth's core and mantle. The core-mantle boundary (CMB) is the most significant internal boundary within our planet, buried at remote depths and probably forever hidden from direct observation; yet this region is very important to our understanding of the dynamic Earth system. The thermal and chemical processes operating near the CMB have intimate relationships to fundamental events in Earth history, such as core formation, and continue to play a major role in the planet's evolution, influencing the magnetic field behavior, chemical cycling in the mantle, irregularities in the rotation and gravitation of the planet, and the mode of thermal convection of the Earth. The D″ region, comprising the lowermost 300 km of the mantle, is known to be highly heterogeneous in material properties on large and small scales, presumably due to thermal and chemical variations, while the outermost core is much more uniform. The substantial knowledge that we now have about this remote region is testimony to the remarkable progress made in geophysical remote sensing, prompted by prodigious increases in data, computational power, and experimental methodologies used to investigate deep earth structure and processes. In the past decade in particular there have been unprecedented multidisciplinary advances in understanding the CMB region, and there are excellent prospects for developing a comprehensive understanding of this region in the next few decades. Fundamental issues yet to be resolved include the causes of layering extensively observed in D″, whether or not downwelling slabs accumulate at the base of the mantle, whether plumes arise from the CMB region to feed hotspots at Earth's surface, the extent of core-mantle chemical reactions, the relative importance of topographic versus electromagnetic coupling across the CMB, and the degree to which mantle structure influences the geomagnetic field and its reversals. This overview highlights the progress and future directions in geophysical investigations of the CMB region.

The core-mantle boundary region

Geomagnetic reversal paths

We consider the problem of constructing a time-dependent map of the magnetic field at the core-mantle boundary. We use almost all the available data from the last 300 years to produce two maps, one for the period 1690-1840 and the other for 1840-1990. We represent the spatial dependency of the field using spherical harmonics, the time dependency using a cubic B-spline basis, and seek the smoothest solutions compatible with the observations. We argue that, for observations from permanent magnetic observatories, the most efficient strategy is to use the first differences of annual means; for satellite data, the most efficient strategy is simply to limit the number of data used so as to minimize any tendency to map the crustal field into the core field. The resulting model fits the observatory data better than any previous model. The resulting time-dependent field map exhibits much of the same structure in the field and its secular variation identified in earlier studies.

Time-dependent mapping of the magnetic field at the core-mantle boundary

This review examines the recent attempts at extracting information on the pattern of fluid flow near the surface of the outer core from the geomagnetic secular variation. Maps of the fluid flow at the core surface are important as they may provide some insight into the process of the geodynamo and may place useful constraints on geodynamo models. In contrast to the case of mantle convection, only very small lateral variations in core density are necessary to drive the flow; these density variations are, by several orders of magnitude, too small to be imaged seismically; therefore, the geomagnetic secular variation is utilized to infer the flow. As substantial differences exist between maps developed by different researchers, the possible underlying reasons for these differences are examined with particular attention given to the inherent problems of nonuniqueness.

Fluid flow near the surface of Earth's outer core

With the aim of producing readable time-dependent maps of the geomagnetic field at the core-mantle boundary, the method of simultaneous stochastic inversion for the geomagnetic main field and secular variation, described by Bloxham (1987), was applied to survey data from the period 1820-1980 to yield two time-dependent geomagnetic-field models, one for the period 1900-1980 and the other for 1820-1900. Particular consideration was given to the effect of crustal fields on observations. It was found that the existing methods of accounting for these fields as sources of random noise are inadequate in two circumstances: (1) when sequences of measurements are made at one particular site, and (2) for measurements made at satellite altitude. The present model shows many of the features in the earth's magnetic field at the core-mantle boundary described by Bloxham and Gubbins (1985) and supports many of their earlier conclusions.

Simultaneous stochastic inversion for geomagnetic main field and secular variation: 2. 1820–1980

The problem of determining the magnetic field originating in the earth's core in the presence of remanent and induced magnetization is considered. The effect of remanent magnetization in the crust on satellite measurements of the core magnetic field is investigated. The crust as a zero-mean stationary Gaussian random process is modelled using an idea proposed by Parker (1988). It is shown that the matrix of second-order statistics is proportional to the Gram matrix, which depends only on the inner-products of the appropriate Green's functions, and that at a typical satellite altitude of 400 km the data are correlated out to an angular separation of approximately 15 deg. Accurate and efficient means of calculating the matrix elements are given. It is shown that the variance of measurements of the radial component of a magnetic field due to the crust is expected to be approximately twice that in horizontal components.

/pdf/accounting-for-crustal-magnetization-in-models-of-the-core-50pdo22twv.pdf

Accounting for crustal magnetization in models of the core magnetic field

Recent studies of the secular variation of the earth's magnetic field over periods of a few centuries have suggested that the pattern of fluid motion near the surface of earth's outer core may be strongly influenced by lateral temperature variations in the lowermost mantle. This paper introduces a self-consistent method for finding the temperature variations near the core surface by assuming that the dynamical balance there is geostrophic and that lateral density variations there are thermal in origin. As expected, the lateral temperature variations are very small. Some agreement is found between this pattern and the pattern of topography of the core-mantle boundary, but this does not conclusively answer to what extent core surface motions are controlled by the mantle, rather than being determined by processes in the core.

Lateral temperature variations at the core-mantle boundary deduced from the magnetic field

The problem of calculating the temporal evolution of both the radial and horizontal poloidal components of a field, given an initial field and the flow and shear, is first considered. Attention is then given to the inverse problem of determining the flow and shear, given an initial field and its temporal evolution. The nonuniqueness inherent in such inversions is discussed, and it is shown that part of the nonuniqueness in the shear is closely related to that in the flow derived from just the radial induction equation.

/pdf/mapping-the-fluid-flow-and-shear-near-the-core-surface-using-54bw45bkvl.pdf

Andrew Jackson

Papers

Fluid flow near the surface of Earth's outer core

Simultaneous stochastic inversion for geomagnetic main field and secular variation: 2. 1820–1980

Accounting for crustal magnetization in models of the core magnetic field

Lateral temperature variations at the core-mantle boundary deduced from the magnetic field

Mapping the fluid flow and shear near the core surface using the radial and horizontal components of the magnetic field