The analytic description of the geomagnetic field at satellite altitudes

The Vanguard 3 (1959η) scalar magnetic values were compared with values of the geomagnetic intensity as computed using various sets of harmonic coefficients (ten in all). Both the arithmetic mean and the ratio of the rms difference (computed intensity minus observed intensity) to the rms of the observed intensity were computed. The ratio was calculated as a function, sequentially, of the number of harmonic coefficients so as to assess the contribution each coefficient made to the total magnetic intensity. Results are that not only can a ‘best’ set of harmonic coefficients be proffered for describing the geomagnetic intensity at satellite altitudes, but also a minimum number of coefficients, depending on the accuracy desired. Also pinpointed were harmonic coefficients in need of revision, in particular, h21 and h31; the maximum contribution to the magnetic intensity by external sources can be estimated to be 10–26 gammas.