Satellite magnetic anomalies with a smooth spectral transition to long wavelengths

Abstract

In most cases, the magnetic field anomalies from satellite data and models are determined by using a cut-off degree that splits the Earth's internal magnetic field (supposed to be of the core and lithospheric origin) into long-wavelength and a short-wavelength parts. This practice allows to effectively compare the signal of both components with other models and controls the signal content as well. In this study an alternative approach is followed to provide the components in long and short wavelengths without cutting the spectrum. Instead, we seek iteratively for the signal loss when the magnetic signal (from degree 1 to the maximum degree of the model) is propagated from the core-mantle boundary to satellite altitude. The problem is generally non-unique, it is required that the signal fit has to follow the slope of the geomagnetic spectrum over the lowest degrees (the degrees least affected by the magnetic lithosphere). The tool operates in the spatial domain – the spectral slope criterion is met after the procedure ends by selecting an appropriate solution. The method can be applied to the vector components individually as soon as they are given in the Earth-fixed Earth-centered frame, for which the integral equation is valid. The obtained magnetic anomalies have different spectral properties because the transition from the long wavelengths to the short wavelengths becomes smooth, although the appropriate number of iterations is questionable. It is found that the long-wavelength signal hands over the magnitude dominance to the short-wavelength part at around degrees 16 to 18.