Geomagnetic secular variation based on spherical harmonic and cross validation analyses of historical and archaeomagnetic data.

Abstract

Historical observations of the geomagnetic field have been combined with archaeomagnetic results in order to produce a model of geomagnetic field changes since 1600 A. D. Cross validation of the historical data indicated the most suitable level of truncation of spherical harmonic modelling of the data to be at the fourth degree. Weighted least squares cubic splines have been used to combine our spherical harmonic analyses carried out at 50 year intervals and to model smooth changes of the historical magnetic field throughout the world. Our model is in broad agreement with previous calculations of the historical main field but differs in some of the details of the non dipole field and the axisymmetric coefficients. An area of sustained anticlockwise looping of the local magnetic field vector has been revealed for the first time in the Indian ocean. The present region of strikingly small secular change and low non dipole field in the Pacific appears to have formed during the last 200 years. Changes in local field intensity have also been modelled throughout the world for the last 400 years. Even the largest of these local intensity changes are significantly smaller than the high frequency palaeointensity changes reported by archaeomagnetists. The balance of energy between the magnetic dipole field and the higher order fields suggested to have been occurring since 1900 A. D. is not found from our analyses for before 1800 A. D.