The effects of the non-uniform distribution of magnetic observatory data on secular variation models

Abstract The effect of the poor spatial distribution of magnetic observatory data on spherical harmonic models of the geomagnetic secular variation is investigated. Values of the secular variation of the north, east and vertical components ( X , Y and Z ) of the geomagnetic field were synthesized from the 1980 IGRF at the positions of 184 currently operating magnetic observatories. To simulate actual observations, Gaussian random noise was added to the synthesized values and randomly selected samples of 75% of the X , Y and Z values were subjected to spherical harmonic analysis. The differences between the results of these analyses and the IGRF are discussed and compared with the input data distributions.