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Traditionally, the main magnetic field and its secular vari- ations were modeled by expanding into spherical harmonics (SH) of the annual means from the global magnetic observa- tory network. Because of averaging within a year, the high- frequency fields generated by currents in the outer mag- netosphere did not virtually affect the annual mean field values. The magnetic surveys from low-altitude satellites have changed the original data both qualitatively and quan- titatively. The instant field measured at a certain point is the sum of the fields caused by intra- and extra-terrestrial sources and having different spectral composition. Conse- quently, modeling becomes a matter of separating the fields of different origin, most of which may be qualified as the main field measuring error. A simple method for elimi- nating the errors is the selection of data based on certain criteria, such as geomagnetic activity indices, solar zenith angle, and parameters of the interplanetary magnetic field. As a result of such selection, most data are rejected, and only the field values, for which the contribution of external sources is negligible, are used for further analysis (Olsen et al., 2002). In this paper, we propose an alternative approach based on the assumption that the fields of different origin caused by different processes in different media must have different temporal and spatial characteristics; i.e., the spatial struc- tures and time variations of such fields do not correlate with each other. If it is true, then the expansion into natural or- thogonal components can be applied to all data without any selection (Langel, 1987). The objective of our present work is to validate the above statement.

The IZMIRAN main magnetic field candidate model for IGRF-10, produced by a spherical harmonic-natural orthogonal component method