Strategies for improved global electromagnetic response estimates.

Abstract

Efforts to make more precise and reliable measurements of the electromagnetic response of the Earth at periods less than a few days are baffled by the contamination of the magnetic variation data by currents which flow in the heterogeneous outer shell of the Earth. In this paper, quantitative estimates of the potential contamination are made by analysing induction in an Earth model which, in addition to the usual steep rise in conductivity at a depth of 500km, also involves a superficial conducting layer of integrated conductance 4,000S. It is concluded that (a) it may be possible, by careful selection of suitable stretches of data from observatories far from conductivity anomalies, to extend global electromagnetic response estimates to periods as short as 8 hours, but that (b) present response estimates in the period range 1-10 days may be contaminated by the currents in the outer shell.These hypotheses are tested by an investigation of the spatial and temporal variations in geomagnetic transfer functions observed at stations in Europe during a three-month period. Two events recorded during that time show distinctly different responses in the period range 1-3 days. The first yields temporally and spatially consistent values of the transfer function linking vertical and horizontal north components. The excitation is most probably of ring current origin, and the bulk of the induced current flows in the deep, spherically symmetric part of the Earth. The transfer functions generated by the second event are also consistent with time, but differ substantially from one obervatory to another. The source of this event is a current system which is spatially non-uniform. The transfer function is partly determined by the configuration of the localised external current system, and partly by the geometry of induced currents in the surface sheet. Both factors give rise to persistent relationships between the magnetic field components, which contaminate the global response estimates.

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

“…The geomagnetic spectrum was divided into frequency bands using the scheme chosen by Banks (1981), and the band centred on the two day period (bandwidth 1.3 to 4.3 days) was selected for detailed investigation. The Fourier estimates in the band were shifted to zero frequency (Banks 1975), to generate a new series of complex demodulates, and resampled at an interval of one day.…”

“…Unfortunately, at periods less than four days, there is ample evidence that the magnetic variation spectrum is the sum of contributions from a number of different sources in the ionosphere and magnetosphere, and that their spatial structure becomes increasingly complex with increasing frequency (Banks 1981). On a global scale, response estimates have been made for the solar daily variation (Malin & Gupta 1977;Winch 19Sl), which is easily recognized by virtue of the line spectrum it generates, and has the advantage of a large amplitude relative to the continuum, so that contamination by other sources is less of a problem.…”

The long-period electromagnetic response of the Earth

“…We can look at this aspect of the data by using the 'complex demodulation' technique. This technique was first presented by Bingham, Godfrey & Tukey (1967) and is reviewed by Banks (1975), who also (Banks 1981) discusses its application to the estimation of 'global' response functions. It allows us the advantages of both time and frequency domain analyses.…”

Geomagnetic variation anomalies in northern England: processing and presentation of data from a non-simultaneous array

“…Both the value and variance of the mean value of the population is a function of geomagnetic latitude and the frequency of the variation field (e.g. Banks 1981). At the latitude of the present study, the data analysis techniques used ensure that, at best, k varies in a statistically random manner over the available data set or, at worst, that the bias introduced by a population with small variance (in k ) is minimized (Beamish 1979).…”

“…In many cases it has become apparent that a number of local field relationships obtained from induction studies can only be interpreted when consideration is given to the wider problem (e.g. Banks 1981).…”