A time‐dependent model of the Earth's magnetic field and its secular variation for the period 1980–2000

Abstract

This study presents an investigation and description of the secular variation of the Earth's magnetic field between 1980 and 2000. A time‐dependent model, C3FM (Continuous Covariant Constrained endpoints Field Model), of the main field and its secular variation between 1980 and 2000 is developed, with Gauss coefficients expanded in time on a basis of cubic B splines. This model is constrained to fit field models from high‐quality vector measurements of Magsat in 1980 and Ørsted in 2000 and to fit both magnetic observatory and repeat station secular variation estimates for the period in between. These secular variation estimates (first time derivatives) are derived from observatories monthly or annual means and repeat station data in order to reduce the contributions of crustal noise, annual, and semiannual variation. On average, the model input consists secular variation estimates of the X, Y, and Z components at 130 locations per month. Treatment of covariance between the different components allows a higher temporal sensitivity of the model, due to the exclusion of some external field variation. The model is computed up to degree and order 15. The model is a useful extension of the hitherto existing time‐dependent description of the secular variation, GUFM which describes the secular variation until 1990. It reveals a short‐term secular variation on subdecadal timescale and has a higher spatial resolution, than previously resolved. The model is also valuable to test the frozen flux hypothesis and to link features of the radial field at the core‐mantle boundary to the geodynamo.