Candidate models for the 1995 revision of IGRF, a worldwide evaluation based on observatory monthly means

Three candidate models for the IGRF 1995 description of the 1995.0 main field are evaluated. These are GS-95 proposed by the Goddard Space Flight Center (U.S.A.), IZ-95 proposed by IZMIRAN (Russia) and UB-95 proposed jointly by the US Navy (U.S.A.) and the British Geological Survey (U.K.). These models are tested against a set of December 1994 and January 1995 preliminary monthly mean values kindly provided to us by 73 observatories distributed throughout the world. This procedure has at least two advantages. First, no secular variation corrections are required for the test to be carried out. Second, all datasets used in the construction of the candidate models include observatory annual means up to at most 1993.5 and share similar links to our independent test data set. We, therefore, expect the test to be reasonably unbiased towards any ofthe models. The procedure has the inconvenience ofbeing sensitive to local crustal anomalies, as pointed out by Langel et al. (1982). Indeed, directly comparing predictions from the candidate models to the observed monthly means reveals little else than a common, correlated, hardly discriminating, disagreement mainly due to crustal anomalies. We repeated the comparison after correcting the monthly means for the crustal anomalies derived by Bloxham and Jackson (1992). This second test was much more significant and showed that the models do a comparable job, UB-95 being marginally better. We recommand equal weighing between the two GS-95 and IZ-95 candidate models, and a slightly larger weight for UB-95.