Space-time modeling of the main magnetic field by combined methods of spherical harmonic analysis and natural orthogonal components

A simple method for obtaining a space-time model of the main magnetic field from the high-precision satellite survey data is described. At the first stage, the CHAMP satellite data for one-day interval are expanded into the spherical harmonics with constant coefficients. This yields a set of daily mean spherical harmonic models (DMSHM) over the survey interval of a few years. At the second stage, the coefficients of this set are used as source data for expansion into the natural orthogonal components (NOC). It is shown that the terms of the NOC series decrease rapidly, and the accuracy of the space-time model of the main geomagnetic field over the time interval under discussion is not worse than the accuracy of the models obtained by traditional methods.