Using observatory data to characterise geomagnetic daily variations

We use data from several geomagnetic observatories to derive a series of Fourier harmonic models of the daily magnetic field variations. We form the models from datasets omprising selected numbers of International Quiet Days from each month, from different years in the solar cycle, and across a range of geomagnetic latitudes. The performances of the models are assessed by comparing their predictions with the input data. We comment on the improvement in daily magnetic field estimates over purely main field models for different seasons, solar cycle phases, and geographic latitudes. We also investigate how the models depend on the number of International Quiet Days used. Comparisons are made with the Comprehensive Model version 4; a global model that, unlike our approach, separates ionospheric sources using satellite data as well as observatory data.