Improvement of satellite data main field modelling through a better separation of internal from external contributions

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Determining the geomagnetic secular variation from satellite magnetic field measurements requires the repeated generation of main field models for different epochs separated by about a month. From the changes in the Gauss coefficients of successive field models the secular variation are derived. This procedure puts high demands on the quality of the models, since any spurious effect may mimic a variation. With the launch of the CHAMP satellite on 15th July 2000 highly accurate magnetic field measurements have become available. The long designed life-time of 5 years and the low polar orbit make this data set particularly useful for secular variation studies. In order to utilise the full accuracy for main field modelling special attention has to be paid to a correct removal of the magnetic contribution from ionospheric currents. Particulary challenging are the high latitudes. Here substantional currents are flowing even on the night side and during magnetically quiet days. In the past little has been done to further improve this situation. In this proposal we plan to model the ionospheric currents in the polar regions by combining ground-based and space-based observations. The aim is to construct a description of ionospheric currents for quiet periods, which is controlled by ground-based and solar wind observations and which can be used to correct the CHAMP magnetic field data prior to main field modelling.