Secular Variation Over EUROPE:A New View

The purpose of modern regional modelling is to describe the geomagnetic field over a restricted region of the Earth, providing a better spatial resolution of the local field for areas of high data density. One currently available method to obtain continuous regional descriptions of main field and secular variation is regularised Spherical Cap Harmonic Analyses, SCHA. The aim of the present work was to model the secular variation field over Europe, the region of highest spatial- temporal coverage with high-quality ground data, by means of SCHA for the past 40 years. In that attempt 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. Firstly, we applied an empirical procedure to minimise the external field contributions to the geomagnetic observatory annual means, as proposed by Verbanac et al., 2007. Further, we applied SCHA with physical regularization and adequate selection of model parametrisation to corrected annual means at 46 European observatory locations and additionally 11 'virtual observatories' chosen to improve the initial data distribution. The quality of the preferred model was verified by considering different criteria: comparisons of time series of model predictions and data and comparisons to a global model. The comparison of the overall rms misfit to all input data from our regional and global model suggests that our new model better describes the secular variation field and gives an improved fit to the features of the data that we assumed to be real internal secular variation. The present study is the first step in obtaining a detailed secular variation behaviour based on European observatory data.