Using observatory data to characterise geomagnetic daily variations

The quiet-time daily variations in the geomagnetic field are known to show strong dependence on latitude, local time, season and solar cycle. In this poster we present preliminary results from surface-harmonic models of these variations derived from ground-based observatories. The data input to the models are hourly means from the five geomagnetically quietest days in June and December 2004 from 98 observatories. These data from each observatory are linearly detrended and Fourier coefficients are fit to them with a fundamental period of 24 hours and minimum period of 6 hours. Surface harmonics up to degree 4 are then fit to the distribution of each Fourier coefficient, separately. The accuracy of the Fourier and spherical harmonic models with respect to the input data is discussed. We also comment on the geographical and seasonal variations in the model.