An examination of observatory biases during the Magsat and طrsted missions

Using magnetic data from the Magsat and طrsted satellite missions, launched in 1979 and 1999, respectively, and data from geomagnetic observatories operating at the same time, we compute and compare observatory biases with the view to identifying changes in the crustal field with time. Where possible, similar selections of satellite and observatory data have been used so as to minimise the contamination of the results with ionospheric and magnetospheric fields. Spherical harmonic models of the core-generated field and large-scale external field are determined from the satellite data and are compared with the ground-based data to give bias values. Results using observatory annual mean values show changes in the biases over the 20-year period which, in some regions, appear to have some consistency. As contamination by magnetic fields with sources in the ionosphere and magnetosphere is a possible cause of these changes, further efforts were made to reduce these effects by using quiet-time selections of observatory hourly mean values. Whilst there remains some indications of regional consistencies in the bias changes, it can only be concluded that real changes in the crustal field over the 20-year period may have occurred. The possibility of some remaining external field contamination in the analysis cannot be completely discounted.