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Ann. Geophys., 24, 567-575, 2006
www.ann-geophys.net/24/567/2006/
© European Geosciences Union 2006
Model modifications in Schumann resonance intensity caused by a localized ionosphere disturbance over the earthquake epicenter
A. P. Nickolaenko1, M. Hayakawa2, M. Sekiguchi2, Y. Ando2, and K. Ohta3
1Usikov Institute for Radio-Physics and Electronics, National Academy of Sciences of the Ukraine, Kharkov, Ukraine
2Department of Electronic Engineering, The University of Electro-Communications, Chofu Tokyo, 182-8585, Japan
3Department of Electronics Engineering, Chubu University, Kasugai Aichi, 487-8501, Japan
Abstract. This paper is a further extension of our latest observations and modeling by
Hayakawa et al. (2005a), in which we discovered the anomalous behavior of Schumann
resonance observed in Japan, in possible association with the Chi-chi
earthquake in Taiwan. Schumann resonance
intensity changes associated with a localized decrease in the lower ionospheric
height over the earthquake epicenter are modeled. The knee model of the vertical
conductivity profile of the ionosphere describes the regular Earth-ionosphere
cavity, and the modified knee model is introduced for the
disturbance. The localized ionosphere modification is of a Gaussian radial
dependence; it has a 1-Mm radius, and the decrease reaches 20 km in the
lower ionosphere height over the epicenter of the earthquake (Taiwan). The
diffraction problem in the Earth-ionosphere cavity with a localized
disturbance is resolved by using the Stratton-Chu integral equation. This
solution is constructed for the case of natural resonance oscillations
driven by independent random sources distributed worldwide. The data of
the Optical Transient Detector (OTD) are used to introduce the source
distribution. A pronounced increase in the intensity of the Schumann resonance
is obtained around the fourth mode frequency (up to 20%) when
thunderstorms are concentrated in Central America. The worldwide
distribution of lightning strokes blurs and slightly reduces the effect
(15% increase in intensity) for the observer in Japan and the localized
nonuniformity positioned over Taiwan. A clear qualitative similarity is
obtained in relation to the experimental data, indicating that records collected in Japan
may be explained by the impact of a localized decrease in the lower
ionosphere over the epicenter of the earthquake. It is admitted that the
assumed conductivity decrease could only be caused by a severe change in the
ionization in the middle atmosphere. It is not in the scope of this paper to
discuss the possible mechanism, but rather to show that a closer and
quantitative agreement with the experiment yields information about the form
and size of the ionospheric modification and about the distribution of global
thunderstorm activity during measurements.
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