engleski

Different approaches for removing the external field influences from the European geomagnetic observatory annual means

Magnetic observatories play a crucial role in monitorning and understanding geomagnetic sec- ular variation. For this reason we need annual means to reflect as much as possible the core field. We investigated data from 46 European geomagnetic observatories collected over 42 years (1960 - 2001). After removing the core field, as predicted by the Comprehensive CM4 model (Sabaka et al., 2004), the apparent link to the solar cycle was noticed as short period variations in the order of ± 10 nT. Although it was recognized in the past (Yukutake, 1987) that observatory annual means are affected by the external fields, no suitable model to reduce these influences exists. Our aim was to develop a method to correct the observatory annual means for external field varia- tions. Further, we systematically studied and explained the different geomagnetic field contributions from sources external to the Earth which are contained in the annual means. First, we tried to minimize these variations by using external field description included in the CM4 model, but were not able to sufficiently reduce the external field contributions. Consequently, we developed an empirical procedure for estimating the external field variations in the data. Taking the advantage of the homogeneity of the external influences in the European region, we constructed templates by median averaging of the data in each year, separately for individual component. After subtracting the templates, the temporal variations were successfully reduced. Further, we investigated the sources and characteristics of various external field contributions and seeked for physical explanations of these signals. For that purpose, the module for external field contributions provided by the POtsdam Magnetic model of the Earth (POMME-2.5) (Maus et al., 2006) was employed. We were focusing on the northward, X, component, since this component is mostly influenced by the external field. The magnetospheric contributions were successfully removed by parameterising the POMME model with the Dst index. The remaining signal was regarded as caused by ionospheric currents. So, the Sq effect estimated by the CM4 model was subtracted and the remained, still significant variation was finally removed with a function prop- erly scaled by Ap and Dst indices. With this procedure we succeed to suppress the external field contributions in the annual means to an uncertainty level of ± 2 nT. The present investigation shows that the external field signal contained in the observatory annual means is significant and their elimination is a prerequisite for obtaining reliable and physically meaningful results when such data are used in studies of the core field and its secular variation.

geomagnetic observatory; annual means; external geomagnetic field; Global Models

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