Global induction and the spatial structure of mid-latitude geomagnetic variations

Abstract

SUMMARY A pre-requisite for more precise estimates of the electromagnetic response of the Earth is a better knowledge of the origin and spatial structure of geomagnetic time variations with periods of a few hours to several years. The period range from four hours to four days is particularly crucial for constraining upper mantle conductivity models, but is difficult to analyse because of overlapping contributions from current systems with different spatial scales. A period band centred on two days has been isolated from two years of hourly mean values recorded at up to 130 observatories. Plots of the coherence between pairs of records of the same component of the field at different observatories, as a function of the station separation, prove to be a valuable diagnostic tool. They show that two groups of sources contribute to this part of the spectrum. One group has a spatial scale in excess of 10 000 km, dominates the geomagnetic north (Xm) component, but produces no east (Ym) component. It can be satisfactorily modelled by a P01 spherical harmonic, and must be generated by fluctuations in the symmetric magnetospheric ring current. The spatial coherence of the second group falls to zero in a distance of 5000–7000 km, and can be predicted by a stochastic source model for which the autocovariance scale length is 7000 km. The spatial structure of the two groups is further investigated using the principal component technique of Egbert & Booker (1989). The spectrum is dominated by the largest eigenvalue, and the eigenfunctions corresponding to this mode show the expected P01 source behaviour. The modes which correspond to the next largest eigenvalues are organized into foci approximately 10 000 km across. The vertical fields associated with these modes are of low amplitude at mid-latitudes, but increase steeply beyond 50d. Estimates of the P01 response based upon the horizontal and vertical fields recorded at a single station will be biased by the contribution from the higher order modes. The magnetic observatory network is incapable of describing them properly on a global basis, and only the European subarray is sufficiently dense to form locally valid response estimates.