Canadian geomagnetic reference field 1985.

Abstract

The new technique of spherical cap harmonic analysis has enabled a model to be constructed of both the main geomagnetic field and its secular variation, that applies to Canada and its adjacent areas over a time range of 25 years. The technique permits the optimum utilization of data over only a portion of the earth's surface, while ensuring that the curl and divergence of the total magnetic field vector are identically zero. With this technique, proper consideration can be taken of altitude variation, a particularly important factor when dealing with satellite data. The effect of crustal anomalies on the determination of the secular variation (SV) model was eliminated by fitting SV independently of main field, and then integrating the SV model. Main field data at epoch 1985.0 were derived by updating Canadian aeromagnetic and Magsat satellite data by means of that integrated SV model. A spherical cap harmonic residual model was constructed from the updated main field data after subtraction of the 1980 International Geomagnetic Reference Field (IGRF 1980) extrapolated to 1985.0. Merging this residual model for 1985.0 and the integrated SV model provided a model with maximum spatial index of 16 for the main field terms at epoch (temporal degree 0) and of 4 for the integrated secular variation terms (temporal degrees 1 to 3), over a spherical cap of half angle 30°. The vector sum of the residual model and the IGRF 1980 at 1985.0 is called the Canadian Geomagnetic Reference Field 1985, and applies from 1960 to 1985 with extrapolation possible to 1990. This reference field has been used to produce the Magnetic Charts of Canada for epoch 1985.0.