Engineering topics

Bloxham, Jeremy

Publications and source records attributed to Bloxham, Jeremy.

11 records

NASA NTRS1992

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

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.

Bloxham, Jeremy↗

NASA NTRS1992

The steady part of the secular variation of the Earth's magnetic field

The secular variation of the Earth's magnetic field results from the effects of magnetic induction in the fluid outer core and from the effects of magnetic diffusion in the core and the mantle. Adequate observations to map the magnetic field at the core-mantle boundary extend back over three centuries, providing a model of the secular variation at the core-mantle boundary. Here we consider how best to analyze this time-dependent part of the field. To calculate steady core flow over long time periods, we introduce an adaptation of our earlier method of calculating the flow in order to achieve greater numerical stability. We perform this procedure for the periods 1840-1990 and 1690-1840 and find that well over 90 percent of the variance of the time-dependent field can be explained by simple steady core flow. The core flows obtained for the two intervals are broadly similar to each other and to flows determined over much shorter recent intervals.

Bloxham, Jeremy↗

NASA NTRS1991

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

Jackson, Andrew↗

NASA NTRS1991

Fluid flow near the surface of earth's outer core

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.

Bloxham, Jeremy↗

NASA NTRS1991

The geomagnetic main field and the geodynamo

Information available on the geomagnetic main field and the geodynamo is presented. Attention is given to the process of mapping the magnetic field, the last version of International Geomagnetic Reference Field Model, and maps of the magnetic field at the core-mantle boundary and their interpretation. Particular consideration is given to the existing geodynamo theories, with special relation given to the Braginsky and Meytlis theory of core turbulence in which the turbulence differs fundamentally from classical turbulence of Kolmogorov type.

Bloxham, Jeremy↗

NASA NTRS1990

On the consequences of strong stable stratification at the top of earth's outer core

The consequences of strong stable stratification at the top of the earth's fluid outer core are considered, concentrating on the generation of the geomagnetic secular variation. It is assumed that the core near the core-mantle boundary is both strongly stably stratified and free of Lorentz forces: it is found that this set of assumptions severely limits the class of possible motions, none of which is compatible with the geomagnetic secular variation. Relaxing either assumption is adequate: tangentially geostrophic flows are consistent with the secular variation if the assumption that the core is strongly stably stratified is relaxed (while retaining the assumption that Lorentz forces are negligible); purely toroidal flows may explain the secular variation if Lorentz forces are included.

Bloxham, Jeremy↗

NASA NTRS1990

Lateral temperature variations at the core-mantle boundary deduced from the 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.

Bloxham, Jeremy↗

NASA NTRS1989

Simultaneous stochastic inversion for geomagnetic main field and secular variation. II - 1820-1980

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.

Bloxham, Jeremy↗

NASA NTRS1989

Simple models of fluid flow at the core surface derived from geomagnetic field models

The fluid flow at the core surface is determined for a given model of temporal magnetic field variation, assuming steady flow and adopting the frozen-flux approximation. Evidence is presented for ageostrophic flow in the form of flow across the geographical equator. A simple pattern of the large-scale toroidal part of the flow is found. The pattern is nearly symmetric about the equator. Changes in the flow pattern are obtained from flow maps covering 5-year intervals beginning in 1915.

Bloxham, Jeremy↗

NASA NTRS1988

The dynamical regime of fluid flow at the core surface

An alternative method for determining the fluid motion immediately beneath the core-mantle boundary is presented which is based on solving the full nonlinear core motions problem. This method is used to examine three dynamical hypotheses about the flow: (1) the steady motions hypothesis; (2) the geostrophic hypothesis; and (3) the toroidal flow hypothesis. Better fits to the field are obtained with the toroidal flows than with geostrophic flows, casting considerable doubt on the validity of the geostrophic hypothesis. Additionally, some indication is found that failure of the frozen-flux approximation, a concomitant assumption, may be a serious obstacle to obtaining reliable maps of the core fluid flow.

Bloxham, Jeremy↗

NASA NTRS1987

Simultaneous stochastic inversion for geomagnetic main field and secular variation. I - A large-scale inverse problem

The method of stochastic inversion is extended to the simultaneous inversion of both main field and secular variation. In the present method, the time dependency is represented by an expansion in Legendre polynomials, resulting in a simple diagonal form for the a priori covariance matrix. The efficient preconditioned Broyden-Fletcher-Goldfarb-Shanno algorithm is used to solve the large system of equations resulting from expansion of the field spatially to spherical harmonic degree 14 and temporally to degree 8. Application of the method to observatory data spanning the 1900-1980 period results in a data fit of better than 30 nT, while providing temporally and spatially smoothly varying models of the magnetic field at the core-mantle boundary.

Bloxham, Jeremy↗