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Ann. Geophys., 19, 171-178, 2001
www.ann-geophys.net/19/171/2001/
© European Geosciences Union 2001
Appraisal of electromagnetic induction effects on magnetic pulsation studies
B. R. Arora1, P. B. V. Subba Rao1, N. B. Trivedi2, A. L. Padilha3, and I. Vitorello3
1Indian Institute of Geomagnetism, Colaba, Mumbai 400 005, India
2Southern Regional Center for Space Research - CRSPE LACESM/CT/UFSM, Santa Maria, RS 97119–900 Brazil
3Instituto Nacional de Pesquisas Espaciais, Sao Jose dos Campos, SP 12201–970, Brazil
Correspondence to: B. R. Arora (bra@iig.iigm.res.in)
Abstract. The quantification of wave
polarization characteristics of ULF waves from the geomagnetic field variations
is done under ‘a priori’ assumption that fields of internal induced currents
are in-phase with the external inducing fields. Such approximation is
invalidated in the regions marked by large lateral conductivity variations that
perturb the flow pattern of induced currents. The amplitude and phase changes
that these perturbations produce, in the resultant fields at the Earth’s
surface, make determination of polarization and phase of the oscillating
external signals problematic. In this paper, with the help of a classical Pc5
magnetic pulsation event of 24 March 1991, recorded by dense network of
magnetometers in the equatorial belt of Brazil, we document the nature and
extent of the possible influence of anomalous induction effects in the wave
polarization of ULF waves. The presence of 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 electrojet. Through numerical calculations, it
is shown that anomalous horizontal fields, that result from induction in the
lateral conductivity distribution in the study region, vary in magnitude and
phase with the polarization of external source field. Essentially, the induction
response is also a function of the period of external inducing source field. It
is further shown that when anomalous induction fields corresponding to the
magnitude and polarization of the 24 March 1991 pulsation event are eliminated
from observed fields, corrected amplitude in the X and Y horizontal components
allows for true characterisation of ULF wave parameters.
Key words. Geomagnetism and paleomagnetism (geomagnetic
induction) – Ionosphere (equatorial ionosphere) – Magnetospheric physics
(magnetosphere-ionosphere interactions)
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