A model of the geomagnetic field for 1970

Abstract

A mathematical description of the geomagnetic field for 1970 has been derived by using 360,000 land, sea, and airborne measurements accumulated since 1939. The model is expressed by 168 spherical harmonic coefficients gnm, hnm to m = n = 12. It was used as the basis for the 1970 edition of the U.S. World Charts of Magnetic Variation.

The normal geomagnetic field in the vicinity of New Zealand

“…For the total force magnetic scalar measurements in the New Zealand region, the Hurwitz et al (1966Hurwitz et al ( , 1974) models for the normal geomagnetic field give the best regional field. For the data considered in this paper (1960)(1961)(1962)(1963)(1964)(1965)(1966)(1967)(1968)(1969) the Hurwitz et al (1966) coefficients have been used but for data recorded since then it is probably better to use the coefficients of Hurwitz et al (1974).…”

“…A comparison of the mean anomaly maps (Figs 1 and 2), with the difference maps for the geomagnetic field models (Figs 4 and 7), shows that the models of Hurwitz et a't. (1966) and Hurwitz et at. (1974) give small mean anomalies and fit the regional variations in the geomagnetic field better than the other models.…”

“…This present study uses these basic data to compare the observed geomagnetic field with the assumed mathematical model to show up any regional bias in the anomalies which may occur. The model is then compared with other commonly used mathematical models of the normal geomagnetic field for the New Zealand regional, the IGRF (IAGA Commission 2 Working Group 4, Analysis of Geomagnetic Field, 1969) and the normal fields of Reilly & Burrows (1973) and Hurwitz et at. (1974), to see if these models provide a closer fit to the regional variations in the observed magnetic field.…”

The effect of electromagnetic attenuation on the spatial spectrum of the geomagnetic field.

“…displays a correlation between the geometrically corrected energy densities and the scale times whose sense, the weaker components of the field having shorter scale times, is compatible with the hypothesis that it is due to a greater electromagnetic attenuation of the more rapidly varying (usually smaller scale) components. This correlation is still present in a time averaged model covering the past 40 years derived from the models of MALIN (1969), MALIN and CLARK (1974), HURWITZ et al (1974), PEDDIE and FABIANO (1976), BARRACLOUGH et a!. (1975), and BARRACLOUGH and BARKER (1980 (Figs.…”

The Use of Geomagnetic Field Models in Magnetic Surveys

“…The complete set of coefficients consists of the Gauss coefficients (g, h) and at least the first-order secular change coefficients (g', h'). Some models are made with a separate determination of secular change, using only repeat station or observatory data (e.g., Hurwitz et al, 1974).…”