Joseph C. Cain

Evaluation of the 1985-1990 IGRF secular variation candidates

Abstract The IGRF secular variation model for 1985–1990 was adopted by the International Association of Geomagnetism and Aeronomy at its Prague meeting in August 1985 as an average of the three candidate models submitted to the committee. We compared the three models at epoch 1985.0 against each other and against a new model based on observatory data available as of July 1, 1985. These comparisons showed that one of the three candidate models disagreed more with the other two and our model, especially in the eastern Pacific. None of the candidate models was seen to respond to a change in the secular variation of the vertical component that appears to have taken place most strongly in the western Pacific area since 1982. The lack of satellite data was seen to be a significant handicap towards prediction of the field change over most of the Earths surface, especially the southern oceans. Maximum errors of any model are estimated to be of the order of 80 nT a−1.

The geomagnetic jerk of 1969 and the DGRFs

Abstract Cubic spline fits to the DGRF/IGRF series indicate agreement with other analyses showing the 1969–1970 magnetic jerk in the h 1 2 and g 0 2 secular change coefficients, and agreement that the h 1 1 term showed no sharp change. The variation of the g 0 1 term is out of phase with other analyses indicating a likely error in its representation in the 1965–1975 interval. We recommend that future derivations of the ‘definitive’ geomagnetic reference models take into consideration the times of impulses or jerks so as to not be bound to a standard 5 year interval, and otherwise to make more considered analyses before adopting sets of coefficients.