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Volume 42, Issue 9
Displaying 1-17 of 17 articles from this issue
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F. J. Lowes, M. A. Shea1990 Volume 42 Issue 9 Pages 959
Published: 1990
Released on J-STAGE: August 11, 2011
JOURNAL FREE ACCESSDownload PDF (64K) -
F. J. LOWES1990 Volume 42 Issue 9 Pages 961-971
Published: 1990
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThis paper discusses the problems which arise in producing a numerical (spherical harmonic) model of the Earth's main magnetic field for a particular epoch when using methods based on least squares. The input data are contaminated by ionospheric, magnetospheric, and crustal fields, and are not simultaneous in time. Some selection of the data is usually needed, and relative weights have to be assigned. Standard least-squares fitting, the problem of truncation, and the modifications used to avoid this and some other problems are outlined. The problem of accurately and unambiguously estimating the uncertainty in the resultant model is emphasised.View full abstractDownload PDF (1122K) -
Joseph C. CAIN, Bill HOLTER, Daan SANDEE1990 Volume 42 Issue 9 Pages 973-987
Published: 1990
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSNumerical tests were made, using least squares fitting of a spherical harmonic model, to a selection of Magsat data to determine the practical limits of this technique with modern computers. The resulting (M102189) model, whose coefficients were adjusted up to n=50, was compared with M07AV6, a previous model which used least squares (on vector data) for coefficients up to n=29, and Gauss-Legendre quadrature (on Z residuals) to adjust the coefficients up to n=63. For the new least squares adjustment to n=50 a condition number of 115 was obtained for the solution matrix, with a resulting precision of 11 significant figures. The M102189 model shows a lower and more Gaussian residual distribution than did M07AV6, though the Gaussian envelope fits to the residual distributions, even for the scalar field, gives “standard deviations” never lower than 6nT, a factor of three higher than the estimated Magsat observational errors. Ionospheric currents are noted to have a significant effect on the coefficients of the internal potential functions.View full abstractDownload PDF (1615K) -
R. A. LANGEL, R. T. BALDWIN, J. R. RIDGWAY, W. Minor DAVIS1990 Volume 42 Issue 9 Pages 989-1000
Published: 1990
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSScalar magnetic field data have been collected from the world's ocean areas by marine research institutes on a routine basis since the early 1950's. About 13 million such data points reside in the collection of the National Geophysical Data Center in Boulder Colorado. These data comprise a valuable source of measurements for modeling the geomagnetic field of the Earth. To derive a suitable reduced data set for such modeling, each ship track is divided into 220km segments. Within each segment the data are tested for magnetic disturbance and for large deviations from the International Geomagnetic Reference Field. The mean of the acceptable data is computed for each segment, together with the standard error of the mean. This results in 24, 243 reduced points. The distribution of the reduced data in time, position and local time is presented and discussed.View full abstractDownload PDF (1076K) -
A ReviewG. V. HAINES1990 Volume 42 Issue 9 Pages 1001-1018
Published: 1990
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSRegional models of the earth's magnetic field have developed considerably since the days of hand contouring. They have incorporated varying levels of mathematical sophistication, from partial “mutual consistency” on a surface to full electromagnetic consistency in three-dimensional space. Each method has its advantages and its limitations. Some of the methods allow for radial variation so that data acquired at different altitudes can be analyzed directly, and so that fields from the resulting models can be calculated at any altitude. For other methods, altitude continuation is theoretically possible but numerically difficult and statistically unreliable. Some methods are limited, for numerical reasons, to small areas or are able to represent only fairly long wavelengths. The different methods also differ greatly with respect to their mathematical and computational complexity, the less complex methods also usually being more subjective.View full abstractDownload PDF (2054K) -
A. DE SANTIS, O. BATTELLI, D. J. KERRIDGE1990 Volume 42 Issue 9 Pages 1019-1036
Published: 1990
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThe technique of spherical cap harmonic analysis, due to HAINES (1985), has been applied to regional field modelling for the Italian area. The data sets used in the analyses include repeat station, magnetic survey, magnetic observatory, and Magsat observations. The spherical cap models derived used the residuals of these observations from the IGRF 1985 as input data. The models are thus essentially models of the crustal field. The truncation level was chosen to enable modelling of features with wavelengths greater than 330km approximately. Initial models were based on the ground data alone. The effect of supplementing the ground data set with Magsat observations was then investigated. The model proposed for adoption as a regional field model for Italy for 1985 is the IGRF 1985 combined with the spherical cap harmonic model derived using all the data sets. The performance of the spherical cap model is discussed in terms of the fit to the residual data, and in relation to previous regional field models for Italy.View full abstractDownload PDF (2175K) -
A DiscussionG. V. HAINES1990 Volume 42 Issue 9 Pages 1037-1049
Published: 1990
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThe choice of basis functions for a given mathematical expansion is critical. It determines whether the expansion is uniformly convergent, whether the expansion can be differentiated termwise, and finally whether this differentiated expansion is uniformly convergent. Basis functions that are chosen with these considerations in mind also provide much faster convergence, and sometimes more meaningful extrapolation, than do those chosen simply on the basis of mean square convergence. These points must be kept in mind when comparing regional models, such as rectangular harmonic and spherical cap harmonic models, whose basis functions have been chosen to give different expansion properties.View full abstractDownload PDF (1072K) -
D. R. BARRACLOUGH1990 Volume 42 Issue 9 Pages 1051-1070
Published: 1990
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSSpherical harmonic models of the main geomagnetic field and its secular variation are used by navigators, exploration geophysicists, drillers of oil wells and by scientists studying the Earth's interior, its ionosphere and magnetosphere. The data on which such models are based must be of the highest quality and must be distributed as homogeneously as possible worldwide. Any relaxation of these criteria will result in significant errors in the models. The spherical harmonic coefficients are derived via the solution of Laplace's equation in spherical coordinates. The resulting expressions can be solved by the method of least squares, either directly or iteratively. If the observations span a long enough time interval, the iterative method can be used to solve simultaneously for the main-field, secular-variation and higher time-derivative coefficients. Data are particular sparse in the southeast Pacific and southern Atlantic and uncertainties of up to 50nT and 10nT/yr in the DGRF model for 1960 are a consequence of this. Errors in predictive models of the secular variation are greater with errors accumulating at a rate of almost 100nT/yr in areas with poor data coverage. Until improved data coverage is available, users should treat all such models with a degree of caution.View full abstractDownload PDF (2000K) -
F. J. LOWES1990 Volume 42 Issue 9 Pages 1071-1078
Published: 1990
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThis paper points out that IGRF-type models of the main geomagnetic field (that coming from electric currents in the Earth's core) are only approximations to the actual field in that they involve low-pass filtering in space and in time, and are inevitably numerically inaccurate. It then discusses the magnetic fields from other sources, in particular from the magnetization of crustal rocks, from electric currents in the ionosphere, from the drift of the charged particles in the radiation belts, and from various magnetospheric processes, attempting to give an indication of typical magnitudes of the various fields.View full abstractDownload PDF (873K) -
Herbert MEYERS, W. Minor DAVIS1990 Volume 42 Issue 9 Pages 1079-1085
Published: 1990
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSNumerical models of the Earth's magnetic field, most commonly in the form of a set of spherical harmonic coefficients, have found applications in fields as disparate as biology, exploration geophysics, land surveying, drilling engineering, navigation, and the study of the Earth's deep interior. A survey, including statistical summaries of these applications are presented. The implications of model abuse will be discussed.View full abstractDownload PDF (787K) -
Thomas WONIK1990 Volume 42 Issue 9 Pages 1087-1097
Published: 1990
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSA compilation of aeromagnetic data for Europe is in progress to produce a map showing crustal anomalies of total intensity for the epoch 1980.0 and the altitude 3000m above m.s.l. and to interpret it in terms of large-scale features in the crust. A report is given of our experience with secular variation and with the reference field which has to be subtracted from the total intensity values to obtain crustal anomalies.
When aeromagnetic surveys are compiled, data from observatories and repeat stations are necessary in order to estimate secular variation with sufficient precision. Data from repeat stations are more detailed and realistic than computed values from global reference field models. The secular variations computed from three different reference field models differ considerably.
The influence of the reference field and its truncation level on the shape of anomalies is examined by calculating differences between the reference fields. The decision to use the DGRF 1980 model for publishing a map showing European crustal anomalies for 1980.0 is based on the fact that this model gives a better approximation of observed values at observatories and repeat stations in Central Europe than two other models. These differences between observed and computed values are partly caused by inhomogeneous input data used for the calculation of reference fields. The input data is inhomogeneous because each observatory publishes their annual mean values based on its own observatory standard and not on the International Magnetic Standard.View full abstractDownload PDF (1248K) -
J. PODSKLAN, M. VÁCZYOVÁ1990 Volume 42 Issue 9 Pages 1099-1102
Published: 1990
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSAnalysis of the different methods of geomagnetic field modelling on the territory of Slovakia is made. Data on the territory of Slovakia was reduced for epoch 1980.5 and compared with values from three models: linear model, IGRF 1980, IGRF, revision 1985. Study of distribution of the deviations from IGRF's for individual points have led to the conclusion there exists a systematic pattern. One of the possible explanations could be a regional anomaly of the geomagnetic field in Europe.View full abstractDownload PDF (287K) -
Luiz Muniz BARRETO1990 Volume 42 Issue 9 Pages 1103-1106
Published: 1990
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSA careful historical analysis of geomagnetic charts shows that their uses have changed between the fifteenth century and the present time. Their exclusive use for navigation has been expanded to a broad application spectrum that includes all orientation purposes and, more recently, prospecting activities and basic geomagnetic research. Such changes have necessitated considerable modifications in chart production, including more accurate data acquisition and analysis, besides the introduction of different methods of graphical representation, in order to express the geomagnetic main field or a local field including anomalies. An adequate choice of scales is essential to discriminate different kinds of field representation. As a consequence of a lack of coordinated work and the stage of scientific development, geomagnetic charts for Latin America do not present a desirable uniformity. For this season, the Pan-American Institute of Geography and History Working Group on Geophysical Maps is attempting to improve geomagnetic charts, through coordinated work on all parts of the subject.View full abstractDownload PDF (494K) -
M. A. SHEA, D. F. SMART1990 Volume 42 Issue 9 Pages 1107-1121
Published: 1990
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSCosmic ray cutoff rigidities, numerically computed using geomagnetic field coefficients, are frequently used as an ordering parameter for cosmic radiation data. Examples of the use of cutoff rigidities as an ordering parameter include latitude surveys, long-term galactic radiation studies, and solar cosmic ray event analyses.View full abstractDownload PDF (1372K) -
E. O. FLÜCKIGER, E. KOBEL1990 Volume 42 Issue 9 Pages 1123-1136
Published: 1990
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSOn the basis of an analysis of the 7/8 December 1982 ground-level cosmic ray event the paper specifies the limitations of any mathematical model of the main geomagnetic field in its use for cosmic ray analyses, presents procedures and problems of combining models of the Earth's internal field with models of the external geomagnetic field, and discusses the significance of the external geomagnetic field for calculations of cosmic ray particle trajectories in near-Earth space.View full abstractDownload PDF (1161K) -
C. P. BAKER, J. E. HUMBLE, M. L. DULDIG1990 Volume 42 Issue 9 Pages 1137-1144
Published: 1990
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThe relationship between primary cosmic ray particles in space and the associated secondary particles observed using ground-based detectors can be described by a set of coupling coefficients. These coefficients depend, among other things, on the direction that the detector is pointing through the geomagnetic field. Along certain directions the coefficients may be highly sensitive to small changes in the field. The accuracy of the field model thus places restrictions on the accuracy of the coupling coefficients. Small errors in the field model can give large relative errors in the coupling coefficients. This will consequently give large relative errors in the interpretation of the space distribution of the primary particles inferred from measured data.View full abstractDownload PDF (661K) -
Patricia H. REIFF1990 Volume 42 Issue 9 Pages 1145-1174
Published: 1990
Released on J-STAGE: September 14, 2010
JOURNAL FREE ACCESSThis paper presents several statistical techniques most commonly used in space physics, including Fourier analysis, linear correlation, auto- and cross-correlation, power spectral density and superimposed epoch analysis, and presents tests to assess the significance of the results. New techniques such as bootstrapping and jackknifing are presented. When no test of significance is in common usage, a plausible test is suggested.View full abstractDownload PDF (2653K)
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