The long-period electromagnetic response of the Earth

The techniques of complex demodulation and spherical harmonic analysis have been used to examine the time-, frequency- and location-dependent properties of the long-period electromagnetic response of the Earth. Fourteen years of daily, three component magnetic field data, recorded by 18 permanent geomagnetic observatories, were used to estimate the response in the period range 2–200 day. It is found that lateral conductivity inhomogeneities within the Earth, notably the oceans, have a considerable effect on the observed response even at these long periods.