Abstract
Extremely low frequency (ELF) electromagnetic waves in the Earth-ionosphere cavity are generated mainly by lightning discharges, originating from the tropical thunderstorm centres. The Earth-ionosphere spherical cavity forms a global resonator for the ELF waves, as predicted by Schumann in 1952. The resonance and the propagation properties of the ELF waves depend on the physical properties of the cavity. The lower ionospheric layers are created by the UV radiation form the Sun. They are modulated by the Sun's 11-year cycle, but sometimes the rapid changes are caused by the solar flares and related sudden ionospheric disturbance (SID) phenomena. During the SID event the development of the ionospheric D layer is observed, which leads to strong damping of the radio waves in the high frequency (HF) band. However, for the ELF waves the attenuation rate decreases significantly and the changes in the resonant frequencies should be observed.
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