On minimizing the external field contributions in annual means of the European geomagnetic observatories

Abstract

The aim of this study is to provide a method for minimizing the external field contributions in the observatory annual means and try to separate, interpret and explain all the different external field signals present in these data. Investigating the European geomagnetic observatory biases over 42 years, considered as contributions of the crustal field, and generally assumed to be constant in time, we noticed the link to the solar cycle, with short period variations in the order of ±10 nT. Developing an empirical method for minimizing the external fields presented in the observatory annual means, we were able to reduce them better than the used of the external field description included in the global Comprehensive Model, CM4 (Sabaka et al., 2004) was. The external field module provided by the POtsdam Magnetic Model of the Earth, POMME-2.5, (Maus et al., 2005) was considered for investigating the sources and characteristics of various external field contributions. We have been able to separate magnetospheric and ionospheric fields, as well as to remove a still significant variation with a function properly scaled by Ap and Dst indices. With this procedure the external field contributions in the annual means are minimized to an uncertainty level of ±2 nT. Our study shows a way to obtain corrected observatory annual means, in order to be used for different studies of the internal geomagnetic field.