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At least 19 recordsLinked to original sources
USGS1962

Analysis of present geomagnetic field for comparison with paleomagnetic results

Both the dipole and nondipole components of the present geomagnetic field are analyzed by calculating the orientation of hypothetical geocentric dipoles which, if acting alone, would produce the present geomagnetic field as observed at points on a grid covering the earth at 10° intervals. The dispersion in direction of these virtual geomagnetic poles due to the nondipole component of thee geomagnetic field (1) is larger in the southern hemisphere than in the northern; (2) increases with increase in latitude; (3) at all latitudes is of the same order of magnitude as the dispersion due to a wobble of several tens of degrees of the main geocentric dipole; (4) is anomalously small over a region of continental dimensions in the central Pacific ocean. The parameters describing these variations of poles are transformed to parameters describing variations of the geomagnetic field for comparison with paleomagnetic data.

Journal of Geomagnetism & Geoelectricity↗
USGS1982

A Proposed Model for the International Geomagnetic Reference Field-1965

A best current model of the main geomagnetic field is presented as a response to a need for an “International Geomagnetic Reference Field”. This model is described by a series of 120 spherical harmonic coefficients and their first and second time derivatives from an epoch 1960.0. It was derived from a sample of all magnetic survey data available from the interval 1900-1964 plus a recent global distribution of preliminary total field observations from the OGO-2 (1965-81A) spacecraft for epoch 1965.8. A duplicate data selection was made and the resulting field model compared with the first to help evaluate the minimum error. It was noted that the root —mean—square difference between the two models was about 30γ in the force components, 0.04 degrees in dip and 0.3 degrees in declination at the earth's surface for 1965.0.

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USGS1969

On the use of magnetic cleaning in paleointensity studies

The use of partial alternating field demagnetization and, to a lesser extent, partial thermal demagnetization has been found useful as a means of removing secondary components of magnetization from rocks used in certain methods of determining paleointensities. During the course of this investigation, which was carried out on lavas erupted during historic times, it was also learned that a partial remanence characterized by relatively low coercivity possessed blocking temperatures throughout the blocking temperature spectrum. On the other hand, a partial remanence characterized by low blocking temperatures possessed only low coercivity.

Hawaii↗
USGS1973

A comparison of three methods of determining geomagnetic paleointensities

Paleointensity studies by the methods of the Thelliers, Wilson, and van Zijl were compared for adjacent specimens from each of five basaltic lava flows. For three of the flows, the actual paleointensity is independently known within ±6%. All specimens had Curie temperatures above 500°C, and those from four of the flows had lower Curie temperatures as well, indicating they probably underwent varying degrees of high-temperature oxidation and disproportionation during initial cooling. These four suffered irreversible changes in their blocking temperature spectra when heated in air above 400 to 500°C. Nonetheless, the paleointensities derived by the Thelliers' method below these temperatures agree with the actual values within the uncertainties. The values obtained by the other methods are not as accurate or reliable. In four out of five lavas, A. F. cleaning of 100 to 200 oe prior to each measurement of remanence in the Thelliers' method degraded the results for determination of paleointensity.

Journal of Geomagnetism & Geoelectricity↗
USGS1974

A discussion of sources and description of the Earth's magnetic field and its secular variations

The problem of collecting data for making geomagnetic charts including secular change is reviewed. Satellite data gives excellent coverage, but the satellite total field intensity data is not sufficient to properly define the field components. In constructing charts, the specific time and space filtering that is used should be specified so the user can make needed corrections in the use of the charts. An attempt should be made to include time variations having periods of one year or greater. In the space domain spherical harmonic coefficients up to degree 12 or 13 should probably be included. Adequate charts n the future will likely require the use of some external coefficients.

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USGS1980

Circular current loops, magnetic dipoles and spherical harmonic analysis.

Spherical harmonic analysis (SHA) is the most used method of describing the Earth's magnetic field, even though spherical harmonic coefficients (SHC) almost completely defy interpretation in terms of real sources. Some moderately successful efforts have been made to represent the field in terms of dipoles placed in the core in an effort to have the model come closer to representing real sources. Dipole sources are only a first approximation to the real sources which are thought to be a very complicated network of electrical currents in the core of the Earth. -Author

Journal of Geomagnetism & Geoelectricity↗
USGS1982

International Geomagnetic Reference Field

In August 1981 the International Association of Geomagnetism and Aeronomy revised the International Geomagnetic Reference Field (IGRF). It is the second revision since the inception of the IGRF in 1968. The revision extends the earlier series of IGRF models from 1980 to 1985, introduces a new series of definitive models for 1965-1975, and defines a provisional reference field for 1975-1980. The revision consists of: (1) a model of the main geomagnetic field at 1980.0, not continuous with the earlier series of IGRF models, together with a forecast model of the secular variation of the main field during 1980-1985; (2) definitive models of the main field at 1965.0, 1970.0, and 1975.0, with linear interpolation of the model coefficients specified for intervening dates; and (3) a provisional reference field for 1975-1980, defined as the linear interpolation of the 1975 and 1980 main-field models. The new models are series of solid spherical harmonics up to and including the tenth degree and order for the main-field models, and up to and including the eighth degree and order for the secular variation model. The models were derived from three sets of proposed models by taking weighted means. The weights were chosen according to the apparent accuracy of the proposed models. A brief history of the IGRF, a review of basic formulas, and a set of world contour maps of the geomagnetic elements based on the IGRF 1980 model are included.

Journal of Geomagnetism & Geoelectricity↗
USGS1982

Assessment of models proposed for the 1981 revision of the IGRF

For the second revision of the International Geomagnetic Reference Field (IGRF), the U. S. National Aeronautics and Space Administration (NASA), the U. K. Institute of Geological Sciences (IGS), and the U. S. Geological Survey (USGS) submitted proposed models of the Earth's main magnetic field at 1965.0, 1970.0, 1975.0, and 1980.0, and its secular variation during 1980-1985. We assessed the proposed models by comparing them with annual mean values from worldwide magnetic observatories, data for 1978-1980 from 63 U. S. magnetic repeat stations, and rates-of-change values for worldwide magnetic observatories for 1965-1985 that were derived from straight lines fitted to annual means for five-year intervals. We also mutually compared the 1980 models.

Journal of Geomagnetism & Geoelectricity↗
USGS1986

Comments on 'Remarks on the secular change in the energy density spectrum of the geomagnetic field' by Joachim Meyer.

Meyer has discussed only the Rn aspect of the Alldredge (1984) paper he is criticising. He has ignored the pictorial demonstration of the need for higher harmonics to properly describe the secular variation field than the main field as demonstrated. This more or less independent demonstration supports the general conclusion of that paper.

Journal of Geomagnetism & Geoelectricity↗
USGS1986

Alternate forms of the associated Legendre functions for use in geomagnetic modeling.

An inconvenience attending traditional use of associated Legendre functions in global modeling is that the functions are not separable with respect to the two indices (order and degree). In 1973 Merilees suggested a way to avoid the problem by showing that associated Legendre functions of order m and degree m+k can be expressed in terms of elementary functions as P m m+k (θ)=sin m (θ)∑ k i=0 a m ki cos(iθ) where a m ki , the constants to be determined, are somewhat analogous to Fourier coefficients. Merilees noted that there are several advantages to using this form, but he also raises a question of precision for degree and order greater than 25. This note calls attention to some possible gains in time savings and accuracy in geomagnetic modeling based upon this form. For this purpose, expansions of associated Legendre polynomials in terms of sines and cosines of multiple angles are displayed up to degree and order 10. Examples are also given explaining how some surface spherical harmonics can be transformed into true Fourier series for selected polar great circle paths.

Journal of Geomagnetism & Geoelectricity↗
USGS1986

Local Magnetic Fields, Uplift, Gravity, and Dilational Strain Changes in Southern California

Measurements of regional magnetic field during gravity, strain and leveling surveys near the San Andreas fault at Cajon, Palmdale and Tejon are strongly correlated with changes in gravity, areal strain, and uplift in these regions during the period 1977-1984. This correlation principally depends on data taken during 1978-79 and 1981-82 when episodes of the ‘Palmdale Uplift’ occurred in this general region. Because the inferred relationships between these parameters are in approximate agreement with those obtained from simple deformation models, the preferred explanation appeals to short-term strain episodes independently detected in each data set. Transfer functions from magnetic to strain, gravity, and uplift perturbations, obtained by least-square linear fits to the data, are -0.98nT/ppm, -0.03nT/μGal, and 9.1nT/m respectively. Tectonomagnetic model calculations underestimate the observed changes and those reported previously for dam loading and volcano-magnetic observations. A less likely alternative explanation of the observed data appeals to a common source of meteorologically generated crustal or instrumental noise in the strain, gravity, magnetic, and uplift data.

Journal of Geomagnetism & Geoelectricity↗
USGS1987

Localized sudden changes in the geomagnetic secular variation.

There is much debate as to whether there was a worldwide geomagnetic jerk in 1969 or 1970. It is agreed that there was an unusual sharp change in the secular variation in the east component, Y, in Europe at that time. This note points out how a localized sudden change in the secular variation pattern of one component in Europe can occur without having any large worldwide effects in any of the components. The accompanying changes in the spherical harmonic coefficients for such a localized change are also discussed.

Journal of Geomagnetism & Geoelectricity↗
USGS1987

Current loops fitted to geomagnetic model spherical harmonic coefficients.

One hundred-sixty circular current loops with radial axes were fitted by least squares to the 899 spherical harmonic coefficients of a 29 th degree model. In the first case, the parameters that were fitted for each loop were the normalized magnetic moment, the distance from the center of the Earth to the current element, the colatitude and E. longitude of the loop axis, and one-half of the central apex angle of the loop. For this case, two of the loops converged near the inner-core outer-core boundary. They accounted for most of the dipolar field. Twenty of the loops, all with much smaller magnetic moments than the two deep loops, converged in the distance range of 0.42 to 0.67 Earth's radius from the center of the Earth. The other 138 loops, after many iterations, were located at distances between 0.81 and 1.0 Earth's radius from the center of the Earth. The loops with radial distances between 0.21 and 0.67 Earth's radius from the center of the Earth are referred to as “core” loops and those at distances greater than 0.81 Earth's radius as “crustal” loops. The spherical harmonic coefficients from these 160 loops, when subtraced from the 899 coefficients of the original model, left a root-mean-square residual of only 0.2nT. A second case was tried which constrained the 138 “crustal” loops to be at 0.996 Earth's radius (25.5km depth). In this case, the root-mean-square residual of the spherical harmonic coefficients from the original model was 0.9nT.

Journal of Geomagnetism & Geoelectricity↗
USGS1987

On regional geomagnetic charts

When regional geomagnetic charts for areas roughly the size of the United States were compiled by hand, some large local anomalies were displayed in the isomagnetic lines. Since the late nineteen sixties, when the compilation of charts using computers and mathematical models was started, most of the details available in the hand drawn regional charts have been lost. One exception to this is the Canadian magnetic declination chart for 1980. This chart was constructed using a 180 degree spherical harmonic model. It managed to show considerable detail, but even more detail might be useful. Suggestions are made about how more detail might be displayed in regional charts when adequate data are available.

Journal of Geomagnetism & Geoelectricity↗
USGS1988

New trend- trigonometric model for interpolation and prediction of the geomagnetic field utilizing the new DGRF models

At the IUGG Assembly at Vancouver during August 1987 new definitive geomagnetic reference field (DGRF) models to degree 10 for 1945, 1950, 1955, and 1960 were adopted by IAGA. Before these new DGRF models were accepted, the author developed a trend and trigonometric model (old trig model) based on the models IGRF 1945, IGRF 1950, IGRF 1955, IGRF 1960, DGRF 1965, DGRF 1970, DGRF 1975, DGRF 1980, and IGRF 1985, which were all approved by IAGA in Prague in August 1985. The old trig model consists of 720 trend and trigonometric coefficients for the calculation of spherical harmonic coefficients (SHC) only to degree eight because the early IGRF models were truncated there. These trend and Fourier sine coefficients can replace the equal number of SHC contained in the 9 DGRF-IGRF models.

Journal of Geomagnetism & Geoelectricity↗
USGS1989

An attempt to obtain a detailed declination chart from the United States magnetic anomaly map

Modern declination charts of the United States show almost no details. Greater detail may be of value to surveyors trying to follow old land deed descriptions, or to pilots of small planes or small pleasure boats operating in inland waterways. It would be extremely expensive to make adequate declination measurements needed for such a chart. It was hoped that declination details could be derived from the information contained in the existing magnetic anomaly map of the United States. This could be realized only if all of the survey data were corrected to a common epoch, at which time a main-field vector model was known, before the anomaly values were computed. Because this was not done, accurate declination values cannot be determined. In spite of this conclusion, declination values were computed using a common main-field model for the entire United States to see how well they compared with observed values. The provisional geomagnetic reference field for 1978.5 was used as the main-field model. The computed detailed declination values were found to compare less favorably with observed values of declination than declination values computed from the IGRF 1985 model itself. This result indicates that the computed anomaly elements or their combination with main-field values cannot be used as accurate anomaly values, but they may be used as an indication of where anomalies probably occur.

Journal of Geomagnetism & Geoelectricity↗
USGS1992

Quiet geomagnetic field representation for all days and latitudes

This paper describes a technique for obtaining the quiet-time geomagnetic field variation expected for all days of the year and distribution of latitudes from a limited set of selected quiet days within a year at a discrete set of locations. We used a data set of observatories operated by Indian and USSR scientists in 1976 and 1977 near 75°E longitude as illustration. Our method relies upon spatial smoothing of the decomposed spectral components. An evaluation of the fidelity of the resulting model shows correlation coefficients usually above 0.9 at the lower latitudes and near 0.7 at the higher latitudes with variations identified as dependent upon season and field element.

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