A preliminary harmonic spline model from Magsat data

Shure, Loren; Parker, Robert L.; Langel, R. A.

doi:10.1029/jb090ib13p11505

Cited by 55 publications

(35 citation statements)

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“…We briefly report on a statistical comparison of the four dynamo simulations with the harmonic spline model PHS80, derived from Magsat data by Shure et al [1985]. However, this is not sufficient to say that the dynamo simulations are not Earth-like, as we have merely made a comparison with certain models that are consistent with real data.…”

Section: Geochemistry Geophysicsmentioning

confidence: 99%

A statistical analysis of magnetic fields from some geodynamo simulations

McMillan

Constable

Parker

et al. 2001

Geochem Geophys Geosyst

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We present a statistical analysis of magnetic fields simulated by the Glatzmaier‐Roberts dynamically consistent dynamo model. For four simulations with distinct boundary conditions, means, standard deviations, and probability functions permit an evaluation based on existing statistical paleosecular variation (PSV) models. Although none closely fits the statistical PSV models in all respects, some simulations display characteristics of the statistical PSV models in individual tests. We also find that nonzonal field statistics do not necessarily reflect heat flow conditions at the core‐mantle boundary. Multitaper estimates of power and coherence spectra allow analysis of time series of single, or groups of, spherical harmonic coefficients representing the magnetic fields of the dynamo simulations outside the core. Sliding window analyses of both power and coherence spectra from two of the simulations show that a 100 kyr averaging time is necessary to realize stationary statistics of their nondipole fields and that a length of 350 kyr is not long enough to fully characterize their dipole fields. Spectral analysis provides new insight into the behavior and interaction of the dominant components of the simulated magnetic fields, the axial dipole and quadrupole. Although we find spectral similarities between several reversals, there is no evidence of signatures that can be conclusively associated with reversals or excursions. We test suggestions that during reversals there is increased coupling between groups of spherical harmonic components. Despite evidence of coupling between antisymmetric and symmetric spherical harmonics in one simulation, we conclude that it is rare and not directly linked to reversals. In contrast to the reversal model of R. T. Merrill and P. L. McFadden, we demonstrate that the geomagnetic power in the dipole part of the dynamo simulations is either relatively constant or fluctuates synchronously with that of the nondipole part and that coupling between antisymmetric and symmetric components occurs when the geomagnetic power is high.

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Section: Geochemistry Geophysicsmentioning

confidence: 99%

A statistical analysis of magnetic fields from some geodynamo simulations

McMillan

Constable

Parker

et al. 2001

Geochem Geophys Geosyst

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“…VOORHIES (1986), and Huang and Harrison (in preparation) argue that the deviation of the locations of null flux lines obtained by Bloxham and Gubbins are incorrectly calculated and consequently, are unreasonably small; therefore, the conclusion of Bloxham and Gubbins that the frozen flux hypothesis should be rejected is very likely to be misleading. SHURE et al (1982SHURE et al ( , 1985 developed harmonic spline models, using a constraint that minimizes the field roughness at the CMB. Although their model may better represent the core field at the earth's surface, it would certainly filter out the small features of the field at CMB.…”

Section: Pattern Of the Field At The Core Mantle Boundarymentioning

confidence: 99%

“…The region where the discrepancy set in was between degrees seven and ten. In later work (SHURE et al, 1985) in which MAGSAT data were analyzed, the discrepancy only became noticeable at degree ten.…”

Section: Introductionmentioning

confidence: 99%

Rates of change of the Earth's magnetic field measured by recent analyses.

Harrison

Huang

1990

J. geomagn. geoelec

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The availability of accurate representations of the Earth's magnetic field for the past 25 years allows us to study the typical rates of change of the field as a function of the degree of the spherical harmonic. It is shown that there is a general increase in the relative rate of change as the degree of harmonic is increased. The analysis was only carried out to degree eight harmonics, because knowledge of the rate of change of the higher degrees is poor. If the projected rate of change continues in the same manner for higher degrees, then it is predicted that harmonics of, say, degree 14 will change at a rate of about 6% per year. This will mean that it should be easy to determine whether harmonics higher than degree fourteen originate in the core or whether they are caused by crustal magnetization. Although westward drift of the field does not necessarily tell us about motions of the core fluid at the core mantle boundary, it is probably true that these motions depend on the same parameters as those used to determine the westward drift. Since the westward drift depends on the longitudinal gradient of the component of field under consideration, it generally depends more on the higher orders of harmonic within each degree than on the lower orders, all other things being equal. In fact, not all other things are equal, because it is shown that in general the power in the higher orders of harmonic is less than the power in the lower orders. Another important factor in determining longitudinal motions is that it is the field at the core mantle boundary which is important, not the field at the Earth's surface. This results in the higher degrees being relatively more important than at the surface of the Earth. But because of the longitudinal differentiation, it turns out that in order to determine a major portion of the longitudinal drift it is necessary to have information about harmonics up to about degree 20. Therefore, with knowledge only of degrees eight or less, we really know rather little about longitudinal motions. Evidence for secular acceleration was found. Not surprisingly, the average rate of secular acceleration

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“…As discussed in Section 2, we can divide the electromagnetic couple (5) into poloidal, advective and leakage couples ra = rp+ rA + r, The observed potential field is usually extrapolated down to the CMB assuming a perfectly insulating mantle Shure, Parker & Langel 1985). But to perform a forward calculation of the electromagnetic couple Table 2).…”

Section: Time-dependent Electromagnetic Coupling: the Forward Problemmentioning

confidence: 99%

Electromagnetic Coupling and the Toroidal Magnetic Field At the Core-Mantle Boundary

Love

Bloxham

1994

Geophysical Journal International

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S U M M A R Y Owing to exchanges of angular momentum between the Earth's fluid outer core and the overlying solid mantle, the Earth's rotational rate fluctuates on periods of a few years to a few decades. However, the mechanism which allows the exchange of angular momentum is not understood. Here we examine the possibility that core-mantle coupling is predominantly electromagnetic and thus responsible for the decadal length of day variations. The electromagnetic couple on the mantle can be divided into poloidal and toroidal parts, and, by requiring continuity of the horizontal component of the electric field at the core-mantle boundary, the toroidal couple can be divided into separate advective and leakage parts. The poloidal couple results entirely from the interaction of the poloidal field with currents induced by its time variation; the advective couple results from the dragging of poloidal field lines through a conducting mantle; and the leakage couple results from the diffusion of toroidal magnetic field from the core's interior into the mantle. The poloidal and advective couples can be estimated by using models of the downward continued poloidal field and models of the core velocity. We find that neither the poloidal couple nor the advective couple exhibit sufficient variability to account for the decadal length of day variations. If this is true, and if core-mantle coupling is indeed predominantly electromagnetic, then most of the variability in the length of day must result from the leakage couple, which, unfortunately, cannot be calculated directly from surface observations. We assume that the horizontal component of the magnetic field is continuous across the core-mantle boundary, that the frozen-flux approximation adequately describes the time dependence of the horizontal component of the magnetic field at the core surface, and that most of this time dependence results from steady core motion. Then by treating the determination of the toroidal field at the core-mantle boundary as an inverse problem, we find that only very strong and spatially complex toroidal field models are consistent with both advection in the core and the decadal length of day variations. We argue that strong toroidal fields are necessary to account for the length-of-day variations since there is significant cancellation when the magnetic stress is integrated over the core-mantle boundary (CMB), the necessary time-dependent torque resulting from the slight and temporary noncancellation of magnetic stress established by a slowly varying and spatially complex toroidal field. But, since our toroidal field models are too strong according to dynamo theory, produce electric fields at the Earth's surface which are stronger than measured values, and produce ohmic heating which either exceeds or contributes an unacceptably large fraction of the Earth's surface heat flow, we deem the toroidal-field models consistent with our analysis of electromagnetic coupling to be physically unreasonable. Thus, we argue that core-mantle coupling is not pr...

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A preliminary harmonic spline model from Magsat data

Cited by 55 publications

References 11 publications

A statistical analysis of magnetic fields from some geodynamo simulations

A statistical analysis of magnetic fields from some geodynamo simulations

Rates of change of the Earth's magnetic field measured by recent analyses.

Electromagnetic Coupling and the Toroidal Magnetic Field At the Core-Mantle Boundary

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