Appraisal of electromagnetic induction effects on magnetic pulsation studies

The quantification of wave polarization characteristics of ULF waves from t he geomagnetic field variations is done under 'a priori' assumption that fi elds of internal induced currents are in-phase with the external inducing f ields. Such approximation is invalidated in the regions marked by large lat eral conductivity variations that perturb the flow pattern of induced curre nts. The amplitude and phase changes that these perturbations produce, in t he resultant fields at the Earth's surface, make determination of polarizat ion and phase of the oscillating external signals problematic. In this pape r, with the help of a classical Pc5 magnetic pulsation event of 24 March 19 91, recorded by dense network of magnetometers in the equatorial belt of Br azil, we document the nature and extent of the possible influence of anomal ous induction effects in the wave polarization of ULF waves. The presence o f anomalous induction effects at selected sites lead to an over estimation of the equatorial enhancement at pulsation period and also suggest changes in the azimuth of ULF waves as they propagate through the equatorial electr ojet. Through numerical calculations, it is shown that anomalous horizontal fields, that result from induction in the lateral conductivity distributio n in the study region, vary in magnitude and phase with the polarization of external source field. Essentially, the induction response is also a funct ion of the period of external inducing source field. It is further shown th at when anomalous induction fields corresponding to the magnitude and polar ization of the 24 March 1991 pulsation event are eliminated from observed f ields, corrected amplitude in the X and Y horizontal components allows for true characterisation of ULF wave parameters.