The analysis of the geomagnetic secular variation

1951 ◽  
Vol 243 (871) ◽  
pp. 525-546 ◽  
Keyword(s):  
Secular Variation ◽  
Graphical Method ◽  
Core Material ◽  
Main Field ◽  
Dynamo Theory ◽  
Geomagnetic Secular Variation ◽  
The Core ◽  
Essential Step ◽  
The Earth ◽  
Thin Current Sheet

After some discussion of the general properties of the secular variation field, a graphical method is described of analyzing it in terms of separate dipoles of arbitrary direction. The analysis shows that the major part of the field for epoch 1922.5 is explained by about twelve vertical dipoles below the surface of the core. The relation of this result with other analyses by McNish (1940) and Bullard (1948) is discussed. The depth of the dipoles confirms that the origin of the secular variation must lie in the core of the earth; because of the high electrical conductivity of the core material it must in fact be due to a thin current sheet at the surface of the core, and this interpretation also gives an explanation for the existence of only vertical sources. The presence of only vertical sources, and in particular the presence of several near the equator, does not support the existence of the toroidal field, which is an essential step in Bullard’s (1949) dynamo theory of the main field.

The westward drift of the Earth's magnetic field

1950 ◽  
Vol 243 (859) ◽  
pp. 67-92 ◽  
Keyword(s):  
Magnetic Field ◽  
Secular Variation ◽  
Outer Part ◽  
Earth’S Magnetic Field ◽  
Dynamo Theory ◽  
The Core ◽  
The Earth ◽  
Earth's Magnetic Field ◽  
Harmonic Analyses ◽  
Westward Drift

The westward drift of the non-dipole part of the earth’s magnetic field and of its secular variation is investigated for the period 1907-45 and the uncertainty of the results discussed. It is found that a real drift exists having an angular velocity which is independent of latitude. For the non-dipole field the rate of drift is 0.18 ± 0-015°/year, that for the secular variation is 0.32 ±0-067°/year. The results are confirmed by a study of harmonic analyses made between 1829 and 1945. The drift is explained as a consequence of the dynamo theory of the origin of the earth’s field. This theory required the outer part of the core to rotate less rapidly than the inner part. As a result of electromagnetic forces the solid mantle of the earth is coupled to the core as a whole, and the outer part of the core therefore travels westward relative to the mantle, carrying the minor features of the field with it.

The secular variation of the earth's magnetic field

1966 ◽  
Vol 62 (4) ◽  
pp. 783-809 ◽  
Author(s):  
D. W. Allan ◽  
E. C. Bullard
Keyword(s):  
Magnetic Field ◽  
Secular Variation ◽  
Analytic Solution ◽  
Conducting Fluid ◽  
Toroidal Magnetic Field ◽  
Main Field ◽  
Geomagnetic Secular Variation ◽  
The Core ◽  
A Cell ◽  
The Magnetic Field

AbstractThe magnetic field observable outside a body of conducting fluid in which field is imbedded may be considerably altered by convection currents in the fluid. One possible explanation of the geomagnetic secular variation foci is that localized convection cells in the earth's core disturb the main field present. An analytic solution for such a process is readily obtained by assuming the form and dimensions for such a cell, and shows that the magnitude of the secular variation cannot easily be explained on these lines without the presence of a subsurface toroidal magnetic field of some hundreds of gauss which is ‘convected through’ the surface of the core.

Constraints on core composition from shock-waves data

1982 ◽  
Vol 306 (1492) ◽  
pp. 37-47 ◽  
Keyword(s):  
Equations Of State ◽  
Liquid Core ◽  
Light Element ◽  
Outer Core ◽  
Atomic Ratio ◽  
Core Material ◽  
Solar Photosphere ◽  
The Core ◽  
The Earth ◽  
Core Composition

Seismic data demonstrate that the density of the liquid core is some 8-10 % less than pure iron. Equations of state of Fe-Si, C, FeS 2 , FeS, KFeS 2 and FeO, over the pressure interval 133-364 GPa and a range of possible core temperatures (3500- 5000 K), can be used to place constraints on the cosmochemically plausible light element constituents of the core (Si, C, S, K and O ). The seismically derived density profile allows from 14 to 20 % Si (by mass) in the outer core. The inclusion of Si, or possibly G (up to 11 %), in the core is possible if the Earth accreted inhomogeneously within a region of the solar nebulae in which a C :0 (atomic) ratio of about 1 existed, compared with a G : O ratio of 0.6 for the present solar photosphere. In contrast, homogeneous accretion permits Si, but not C, to enter the core by means of reduction of silicates to metallic Fe-Si core material during the late stages of the accumulation of the Earth. The data from the equation of state for the iron sulphides allow up to 9-13 % S in the core. This composition would provide the entire Earth with a S:Si ratio in the range 0.14-0.3, comparable with meteoritic and cosmic abundances. Shock-wave data for KFeS 2 give little evidence for an electronic phase change from 4s to 3d orbitals, which has been suggested to occur in K, and allow the Earth to store a cosmic abundance of K in the metallic core.

Modelling the core magnetic field of the Earth

1982 ◽  
Vol 306 (1492) ◽  
pp. 179-191 ◽  
Keyword(s):  
Magnetic Field ◽  
Spherical Harmonics ◽  
Secular Variation ◽  
Long Wavelength ◽  
Core Field ◽  
The Core ◽  
The Earth ◽  
High Degree ◽  
Current Systems ◽  
The Magnetic Field

The magnetic field generated in the core of the Earth is often represented by spherical harmonics of the magnetic potential. It has been found from looking at the equations of spherical harmonics, and from studying the values of the spherical harmonic coefficients derived from data from Magsat, that this is an unsatisfactory way of representing the core field. Harmonics of high degree are characterized by generally shorter wavelength expressions on the surface of the Earth, but also contain very long wavelength features as well. Thus if it is thought that the higher degree harmonics are produced by magnetizations within the crust of the Earth, these magnetizations have to be capable of producing very long wavelength signals. Since it is impossible to produce very long wavelength signals of sufficient amplitude by using crustal magnetizations of reasonable intensity, the separation of core and crustal sources by using spherical harmonics is not ideal. We suggest that a better way is to use radial off-centre dipoles located within the core of the Earth. These have several advantages. Firstly, they can be thought of as modelling real physical current systems within the core of the Earth. Secondly, it can be shown that off-centred dipoles, if located deep within the core, are more effective at removing long wavelength signals of potential or field than can be achieved by using spherical harmonics. The disadvantage is that it is much more difficult to compute the positions and strengths of the off-centred dipole fields, and much less easy to manipulate their effects (such as upward and downward continuation). But we believe, along with Cox and Alldredge & Hurwitz, that the understanding that we might obtain of the Earth’s magnetic field by using physically reasonable models rather than mathematically convenient models is very important. We discuss some of the radial dipole models that have been proposed for the nondipole portion of the Earth’s field to arrive at a model that agrees with observations of secular variation and excursions.

On the determination of the size of the Earth’s core from observations of the geomagnetic secular variation

1981 ◽  
Vol 374 (1756) ◽  
pp. 15-33 ◽  
Keyword(s):  
Magnetic Field ◽  
Secular Variation ◽  
Magnetic Data ◽  
Time Interval ◽  
Perfect Conductor ◽  
Geomagnetic Secular Variation ◽  
The Core ◽  
First Case ◽  
Fractional Error ◽  
The Magnetic Field

When the magnetic field of a planet is due to self-exciting hydromagnetic dynamo action in an electrically conducting fluid core surrounded by a poorly-conducting ‘mantle', a recently proposed method (Hide 1978,1979) can in principle be used to find the radius r c of the core from determinations of secular changes in the magnetic field B in the accessible region above the surface of the planet, mean radius r s , with a fractional error in r c of the order of, but somewhat larger than, the reciprocal of the magnetic Reynolds number of the core. It will be possible in due course to apply the method to Jupiter and other planets if and when magnetic measurements of sufficient accuracy and detail become available, and a preliminary analysis of Jovian data (Hide & Malin 1979) has already given encouraging results. The ‘magnetic radius’ ̄r̄ c of the Earth’s molten iron core has been calculated by using one of the best secular variation models available (which is based on magnetic data for the period 1955-75), and compared with the ‘seismological’ value of the mean core radius, r c = 3486 ± 5 km. Physically plausible values of r̄ c are obtained when terms beyond the centred dipole ( n = 1) and quadrupole ( n = 2) in the series expansion in spherical harmonics of degree n = 1,..., ^ n ,..., n * are included in the analysis (where 2 ≼ ^ n ≼ n *≼ ∞). Typical values of the fractional error ( r̄ c - r c ) / r c amount to between 0.10 and 0.15. Somewhat surprisingly, this error apparently depends significantly on the value of the small time interval considered; the error of 2% found in the first case considered, for which ^ n — n * = 8 and for the time interval 1965-75, is untypically low. These results provide observational support for theoretical models of the geomagnetic secular variation that treat the core as an almost perfect conductor to a first approximation except within a boundary layer of typical thickness much less than 1 km at the core-mantle interface.

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