Wavelet characterization of external magnetic sources as observed by CHAMP satellite: evidence for unmodelled signals in geomagnetic field models

We apply continuous wavelet transform technique to the full decade (2001-2010) of CHAMP vector magnetic data from 55417 tracks, to search for evidences of external source field signatures that are either misunderstood or ignored in modern magnetic field models. We show that satellite magnetic time-series, after subtracting the main and lithospheric field contributions, exhibit several external source signals. Other than the diurnal, 27-day, and annual periodicities, we also have observed the 210-day periodicity, in the external magnetic field. Central to these observations is the local time dependency of 27-day variations, which suggests that the purely zonal source model, generally considered for the estimation of electromagnetic induction response is inadequate. We discuss the origin and characteristics of 210-day periodicity, although its geophysical significance needs to be fully ascertained. A better understanding of the external fields as seen at satellite altitude is a prerequisite for an optimum exploitation of Swarm multi-satellite mission, which is scheduled for launch in 2013.