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European geomagnetic observatories: quality and role in understanding the secular variation

When trying to model the geomagnetic main field and its secular variation at restricted European region by means of Spherical Cap Harmonic Analyses (SCHA), we mainly encountered two major difficulties: i) data are not representing the magnetic field aimed to be modeled and ii) known drawbacks of the modeling technique. This was the motivation for performing two detailed studies, that we present. First, we investigated the European geomagnetic observatory biases over 42 years (1960 - 2001), considered as contributions of the crustal field, and generally assumed to be constant in time. To estimate these biases, we compared observatory annual means to predictions given by the continuous CM4 model and to four other satellite based core field models for different epochs. Comparison of the processed data set with the synthetic values given by the CM4 model, made it possible to highlight the magnetic contributions which remain in real data when only the core field estimated by CM4 is removed. A large part of the found offsets are rather constant shifts, being mainly the crustal field signature. However, long-term trends are detected in many biases. These trends are unlikely to originate from instrumental drift or data errors, being systematic over long time interval. Solar-cycle related external fields are clearly present in the residuals. Although well-known, no suitable model to minimise them exists. We suggest an empirical approach to minimise these influences. Further, we applied SCHA with physical regularization to synthetic series obtained from the CM4, at 46 European observatory locations and additionally 11 'virtual observatories' chosen to improve the initial data distribution. We show that an adequate selection of model parametrisation together with the physical regularization allow that model smoothness and misfit are those required by the data themselves. Models computed for different epochs satisfy the proposed validation criteria, underlining the reliability to compute stable models over the whole considered time span. This study is the first step in obtaining a detailed secular variation behaviour based on European observatory data what will open a way to use regional modelling by means of SCHA for studying the trends in biases further.

secular variation; Spherical Cap Harmonic Analyses; CM4 Global Model; geomagnetic observatory

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