Abstract
The use of complex demodulation techniques, where a frequency band of interest in a given time series is shifted to zero and then low pass filtered, is discussed. Instantaneous values of signal phase and amplitude are derived from the complex demodulates and cross spectra can be estimated from averages of their products. The technique is discussed with particular reference to the geomagnetic field; the instantaneous properties of the magnetic field fluctuations yielding information concerning the non stationary properties of hydromagnetic waves (the wave polarization parameters). Cross spectral determinations can be used to produce transfer functions which enable the response of the Earth to be estimated for variations of different frequencies. Complex demodulation enables a selection criteria to be applied so that more reliable values of the transfer function may be computed.
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