A time{dependent model of the Earth’s magnetic fleld and its secular variation for the period 1980 to 2000

[1] 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 Orsted 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.

[1]  J. Bloxham.  The determination of fluid flow at the core surface from geomagnetic observations , 1988 .

[2]  Mioara Mandea,et al.  The 9th-Generation International Geomagnetic Reference Field , 2003 .

[3]  Jeremy Bloxham,et al.  Time‐dependent mapping of the magnetic field at the core‐mantle boundary , 1992 .

[4]  George E. Backus,et al.  Bayesian inference in geomagnetism , 1988 .

[5]  John Leif Jørgensen,et al.  Ørsted satellite captures high‐precision geomagnetic field data , 2001 .

[6]  Walter H. F. Smith,et al.  Free software helps map and display data , 1991 .

[7]  David Gubbins,et al.  Can the Earth's magnetic field be sustained by core oscillations? , 1975 .

[8]  Gwilym M. Jenkins,et al.  Time series analysis, forecasting and control , 1972 .

[9]  Carl de Boor,et al.  A Practical Guide to Splines , 1978, Applied Mathematical Sciences.

[10]  Nils Olsen,et al.  A model of the geomagnetic field and its secular variation for epoch 2000 estimated from Ørsted data , 2002 .

[11]  Matthew R. Walker,et al.  Four centuries of geomagnetic secular variation from historical records , 2000, Philosophical Transactions of the Royal Society of London. Series A: Mathematical, Physical and Engineering Sciences.

[12]  F. Lowes.  Mean‐square values on sphere of spherical harmonic vector fields , 1966 .

[13]  T. Gold,et al.  On the Generation of Magnetism by Fluid Motion , 1950 .

[14]  Paul H. Roberts,et al.  On Analysis of the Secular Variation , 1965 .

[15]  Jeremy Bloxham,et al.  Geomagnetic field analysis—III. Magnetic fields on the core—mantle boundary , 1985 .

[16]  C. Reigber,et al.  CHAMP mission status , 2002 .

[17]  Nils Olsen,et al.  A comprehensive model of the quiet‐time, near‐Earth magnetic field: phase 3 , 2002 .

[18]  Nils Olsen,et al.  Extending comprehensive models of the Earth's magnetic field with Ørsted and CHAMP data , 2004 .

[19]  J. Cain,et al.  Derivation of a geomagnetic model to n=63 , 1989 .

[20]  Nils Olsen,et al.  Core surface flow modelling from high‐resolution secular variation , 2006 .

[21]  P. Kotzé,et al.  The time-varying geomagnetic field of Southern Africa , 2003 .

[22]  M. Dowson,et al.  Notes on geomagnetic repeat measurements at Grytviken, April 1987 , 1988 .

[23]  Mioara Mandea,et al.  A geomagnetic jerk for the end of the 20th century , 2000 .

[24]  S. Macmillan,et al.  A geomagnetic jerk for the early 1990's , 1996 .

[25]  Gauthier Hulot,et al.  Detection of geomagnetic jerks using wavelet analysis , 1995 .

[26]  Jeremy Bloxham,et al.  The treatment of attitude errors in satellite geomagnetic data , 1996 .