Numerical experiments in geomagnetic modeling.

Cain, J. C.; Holter, Bill; Sandee, D.

doi:10.5636/jgg.42.973

Cited by 23 publications

(30 citation statements)

References 15 publications

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“…The Magsat map, termed M102389, [Cain et al, 1990] estimated static fields to degree and order 49 while the Ørsted map estimates static fields to degree and order 29. The Ørsted map uses techniques described in Olsen et al [2000].…”

Section: New Satellite Magnetic Field Observationsmentioning

confidence: 99%

The southern edge of cratonic North America: Evidence from new satellite magnetometer observations

Purucker

Langlais

Olsen

et al. 2002

Geophysical Research Letters

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[1] A global model is developed for both induced and remanent magnetizations in the terrestrial lithosphere. The model is compared with, and well-described by, Ørsted satellite observations. Interpretation of the observations over North America suggests that the large total field anomalies, associated with spherical harmonic degrees 15 -26 and centered over Kentucky and the south-central United States, are the manifestations of the magnetic edges of the southern boundaries of cratonic North America. The techniques and models developed here may be of use in defining other cratonic boundaries.INDEX TERMS: 1545 Geomagnetism and Paleomagnetism: Spatial variations (all harmonics and anomalies); 1219 Geodesy and Gravity: Local gravity anomalies and crustal structure; 7218 Seismology: Lithosphere and upper mantle; 9350 Information Related to Geographic Region: North America Background[2] The launch of the Ørsted high-precision geomagnetic field satellite [Neubert et al., 2001] has invigorated efforts to understand the magnetic field of the earth's lithosphere. Early attempts [Langel and Hinze, 1998] to model the lithospheric field relied on forward and inverse approaches over local regions. After the realization that much of the lithospheric magnetic signal might be obscured by overlap with the long-wavelength magnetic field from the core [Meyer et al., 1985], recent work has explored the potential of global forward [Cohen and Achache, 1994; Dyment and ArkaniHamed, 1998] and inverse models [Purucker et al., 1998]. Our work elaborates on but differs from previous work which 1) included remanent magnetizations associated only with the Cretaceous Quiet Zones [Cohen and Achache, 1994;Purucker et al., 1998], 2) did not consider induced magnetizations at all [Dyment and Arkani-Hamed, 1998], 3) used a model of induced magnetization that had fewer geologic and geophysical inputs [Purucker et al., 1998]. A New Global Magnetization Model[3] The global model of induced magnetization is based on an estimate of the volume of the magnetic crust and its magnetic susceptibility. We assume that induced magnetizations are restricted to the crust [Wasilewski and Mayhew, 1992] and utilize a global seismic tomography model [Nataf and Ricard, 1996] for estimating crustal thickness. The model also contains a tectonicbased subdivision of the crust into three categories, each of which has an associated geotherm. These geotherms, when coupled with an assumption about the magnetic mineral(s) responsible for the bulk of the magnetization, allow for the calculation of a depth to the Curie isotherm. We assume here that the magnetic mineral is magnetite or low-Ti magnetite. The magnetic layer thickness is calculated as the thickness of the igneous crust above the magnetite Curie isotherm. We utilize a sediment thickness model [Laske and Masters, 1997] to account for the presence of effectively non-magnetic sediment or sedimentary rock which serves to decrease the effective magnetic layer thickness. Although we calculate our induced magnetization mod...

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Section: New Satellite Magnetic Field Observationsmentioning

confidence: 99%

The southern edge of cratonic North America: Evidence from new satellite magnetometer observations

Purucker

Langlais

Olsen

et al. 2002

Geophysical Research Letters

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“…Of all the satellite altitude anomaly maps we tested using the method described in the next section (Cain et al, 1990;Langel, 1990;Ravat et al, 1995;Alsdorf et al, 1998), the Magsat magnetic anomaly map based on the model of Cain et al (1990) (Figure 2b) was most similar to the aeromagnetic map in all numerical comparisons. This map comprises scalar values computed from spherical harmonic coefficients of degrees 14 to 49 at 400 km.…”

Section: Satellite Magnetic Anomaly Mapmentioning

confidence: 99%

“…Because the satellite data are collected and processed more homogeneously than aeromagnetic data, the limited areas of larger differences indicate that problems may exist in or near edges of some of the aeromagnetic surveys. The results from the quantitative comparisons of the EPB aeromagnetic data and the Magsat magnetic anomaly data of Cain et al (1990) are presented in Table 2. The slopes (from a linear re- gression of observed and inverted anomalies) and correlation coefficients close to unity indicate a similarity of their amplitudes and a high degree of association between the two data sets, respectively.…”

Section: Inverse Problem and Weighting Of Data Sets Matricesmentioning

confidence: 99%

“…The coherence measures the amount of association between two data sets in a band of wavenumbers about a central wavenumber (Kanasewich, 1981). The coherence and power between the observed anomalies [Cain et al (1990) model and the EPB aeromagnetic anomalies] and the anomalies computed from the joint inversion are shown in Figures 5a and 5b, respectively. At the relevant wavenumbers, the association is strong for the comparisons at both altitudes, as indicated by coherence values of >0.80.…”

Section: Wavenumber-domain Comparisons Between the Satellite And Aeromentioning

confidence: 99%

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Compatibility of high-altitude aeromagnetic and satellite-altitude magnetic anomalies over Canada

et al. 2002

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Results from equivalent-source distributions derived jointly from high-altitude (average 4 km) aeromagnetic and Magsat-derived (average 400 km) magnetic anomalies over Canada indicate that long-wavelength components (500–2500 km) in these fields are extremely compatible with one another (with a correlation coefficient of 0.95). The jointly estimated anomaly field at the earth's surface can be used as a long-wavelength adjustment surface for regional near-surface magnetic anomaly compilations and in assessing the performance of other downward-continuation techniques. Because near-surface anomalies are not available over all regions of the world, we compare the jointly estimated anomaly field to the results of two different downward-continuation techniques: the evaluation of anomalies at the earth's surface from spherical harmonic coefficients derived from satellite-altitude data and the use of downward-continuation methods based on harmonic splines. Numerical and visual comparisons of these downward- continued fields with the jointly estimated anomaly field from the equivalent-source method indicate they are well correlated and could provide a useful method of deriving long-wavelength leveling surfaces for regional and worldwide magnetic anomaly maps.

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“…(of course these topics are not independent). The problems which arise in producing numerical models of the Earth's main-field and of its time variation (secular-variation) have been outlined (e.g., Cain et al, 1990;Barraclough, 1990;Lowes, 1990). True models accuracies are function of the spatial and temporal distribution of the observatories data employed in the model derivations (Barraclough, 1978;Langel et al, 1982;Barker and Barraclough, 1985;Langel et al, 1989).…”

Section: Introductionmentioning

confidence: 99%

Geographical Distribution of Magnetic Observatories and Field Modelling.

Alexandrescu

Duyen

Mouël

1994

J. geomagn. geoelec

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This study presents some characteristics of geomagnetic models computed from magnetic observatory data. We first analyse the covariance matrix of the Gauss coefficients for different data distributions and different truncation degrees of the spherical harmonic expansion. We then compute the ratio R of the mean square value of the diagonal terms of this covariance matrix to the mean square value of its off-diagonal terms. Finally, we estimate the accuracy of two main-field models, computed using annual mean values from 132 magnetic observatories and two weights series. The effect of the uneven distribution of observatories on global modelling of the geomagnetic field and its secular-variation is underlined. The accuracy of such models is good enough in the areas where the density of observatories is high enough (Europe, North America, East Asia), but it decreases dramatically in poorly covered areas.

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Numerical experiments in geomagnetic modeling.

Cited by 23 publications

References 15 publications

The southern edge of cratonic North America: Evidence from new satellite magnetometer observations

The southern edge of cratonic North America: Evidence from new satellite magnetometer observations

Compatibility of high-altitude aeromagnetic and satellite-altitude magnetic anomalies over Canada

Geographical Distribution of Magnetic Observatories and Field Modelling.

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