Evaluation of models proposed for the 1991 revision of the International Geomagnetic Reference Field

The 1991 revision of the International Geomagnetic Reference Field (IGRF) comprises a definitive main-field model for 1985.0, amain-field model for 1990.0, and a forecast secular-variation model for the period 1990-1995. The five 1985.0 main-field models and five 1990.0 main-field models that were proposed have been evaluated by comparing them with one another, with magnetic observatory data, and with Project MAGNET aerial survey data. The three secular-variation models that were proposed have also been compared with one another, and with estimates of recent rates of change at the observatories. The comparisons indicate that the main-field models proposed by IZMIRAN, and the secular-variation model proposed jointly by the British Geological Survey and the U. S. Naval Oceanographic Office, should be assigned relatively lower weight in the derivation of the new IGRF models.