Understanding models of the geomagnetic field by Fourier analysis.

Fourier spectra of X, Y, and Z components for both latitudinal and longitudinal circular profiles are deduced analytically from a n*=64 spherical harmonic model of the internal geomagnetic field. Expressions and numerical results demonstrate that the spectra behave differently for different field components and different profiles. Simplified anomaly model studies indicate that, despite being much less in amplitude with respect to the spectra of the core field, the spectra with long wavelengths of crustal field are generally more significant than those with short wavelengths of the same source. Wavelength should not be used as a criterion to discuss the depth of the field sources. The crustal field may be seriously distorted by high-pass filtering, whereas truncation at the commonly accepted level of spherical harmonic model of the core field has little effect on localized patterns of the anomalies.