GEOMAGNETIC MAIN-FIELD MODELING WITH POGS SATELLITE DATA

Models of the geomagnetic main field of degree and order 13 are constr ucted for epoch 1992.5. These are based on combinations of (quiet-time , low activity) Polar Orbiting Geomagnetic Survey (FOGS) total intensi ty data, and various near-surface, largely vector, measurements. We co nsider the significance in the fit of models to data of: (a) the spati al sampling interval; (b) the estimated ionospheric field corrections at FOGS satellite altitude produced by Quinn et al. (1995); (c) the de gree and order one external field coefficients; and (d) the relative p roportions of POGS and near-surface data. At this degree and order it is found that: (a) the main-field model accuracy is relatively insensi tive to the average sampling interval over a 15:1 range of intervals; (b) the most accurate models are those in which the estimated ionosphe ric field corrections are not included; (c) the external dipole term i s of the order of +20 nT and hence has a marginal impact on model accu racy at the Earth's surface; and (d) a relatively high ratio, of order 4:1, of FOGS to surface data optimizes model accuracy at both FOGS sa tellite altitude and ground level, although only a low ratio of FOGS t o surface data, of order 1 :9, is required for high model accuracy at the Earth's surface alone. Overall, the most accurate main-field model produces spot values at the surface of the Earth with residual standa rd deviations between 96 nT and 120 nT, depending on component. An est imate of a typical ''true'', or absolute, model accuracy is also given , by comparing computed values, from a simplified model, with measured data from an independent near-surface data set. It is concluded that the absolute model accuracy at the Earth's surface is of the order of twice the accuracy of the model fit to its input data, again depending on component.