THE 1995 REVISION OF THE JOINT US UK GEOMAGNETIC-FIELD MODELS .1. SECULAR VARIATION/

We present the methods used to derive mathematical models of global se cular variation of the main geomagnetic field for the period 1985 to 2 000. These secular-variation models are used in the construction of th e candidate US/UK models for the Definitive Geomagnetic Reference Fiel d at 1990, the International Geomagnetic Reference Field for 1995 to 2 000, and the World Magnetic Model for 1995 to 2000 (see paper II, Quin n et al., 1997). The main sources of data for the secular-variation mo dels are geomagnetic observatories and repeat stations. Over the areas devoid of these data secular-variation information is extracted from aeromagnetic and satellite data. We describe how secular variation is predicted up to the year 2000 at the observatories and repeat stations , how the aeromagnetic and satellite data are used, and how all the da ta are combined to produce the required models.