An Evaluation of a Regional Orthogonal Field Model for Canada with Respect to the Canadian Geomagnetic Reference Field

Abstract

Regional spatial-temporal models of the geomagnetic field have been produced by a variety of techniques. Recently, spherical cap harmonic analysis has gained wide acceptance as a means of modelling the magnetic field over a portion of the globe. Time variations of the field are commonly represented by polynomials or trigonometric series. The Canadian Geomagnetic Reference Field for 1995 (CGRF) is an example of a model produced in this manner. There are, however, other techniques that may offer advantages for some applications. The method of natural orthogonal components is one of these. To demonstrate this method, a spatial-temporal model of the geomagnetic field changes over Canada (ROM-Canada) has been produced using North American observatory annual means and Canadian repeat station data. These data were fitted with rectangular Legendre polynomials using natural orthogonal components as the temporal functions. Overall, total-intensity first-differences computed from the CGRF and ROM-Canada model the observed first-differences with comparable accuracy. ROM-Canada appears to be somewhat more accurate in the centre of the modelling area, but less accurate near the edges. Moreover, the ROM-Canada models of the vector components are less accurate than the CGRF.