| The Schumann resonances are global electromagnetic resonances in the Earth-ionosphere cavity, with a set of distinct spectrum peaks at extremely low frequencies (ELF) around8,14,20,26Hz, excited primarily by global lightning discharges. It is necessary to research into the mechanism of Schumann resonance for the sake of tracking global lightning activity and Schumann resonance can also be used to monitor climate change and extreme weather as a result of the connection between lightning activity and the Earth’s climate. The response of diurnal variations Schumann resonance signals to global lightning activity in the Earth-ionosphere cavity is focus on in this paper. Based on the theoretical model of ELF radio propagation in the Earth-ionosphere cavity, using the experimental ELF data measured at multi-stations and OTD/LIS satellite lightning data, the diurnal variations of dynamic spectrum in the ELF band between5-30Hz is been simulated and compared to the experimental results. The analysis of resonance of Schumann resonance to lightning activity in different areas all over the world has been presented., and the mechanism of the local terminator effect in Schumann resonance signals has also been discussed. In this paper, the main results are as follows:(1) Based on the experimental ELF data measured at Mizpe-Roman site in Israel, detailed statistical analysis of the magnetic component of ELF background noise in time domain and frequency domain has been presented. The results show that ELF signals in time domain follows the normal distribution very well, the mean and variance of the ELF signals fit the mathematical expectation and the standard deviation of the normal distribution. The peak frequencies of the first three modes of the Schumann resonance power spectrum remain constant besides tiny discrepancies between the north-south magnetic component and the east-west magnetic component of power spectrum. Diurnal deviations from the theoretical values of the resonance frequencies of the first three modes (8,14,20Hz) are less than0.3Hz,0.55Hz,0.6Hz, respectively. The dynamic spectrum in the ELF band shows that two power spectrum peaks at07-09UT and19-22UT at the fundamental Schumann resonance mode for the north-south horizontal magnetic field power spectrum, produced by lightning activities in Southeast Asia and the America respectively, and a sole prominent peak at12-15UT appears at all the first three modes for the east-west horizontal magnetic field power spectrum, which related to "the Africa chimney". (2) According to the theoretical model of ELF radio propagation model in uniform Earth-ionosphere cavity, taking the OTD/LIS satellite lightning data as input, the dynamic spectrum of magnetic component of SR band has been simulated and compared to the experimental results. The consistency between the simulation and measurements illustrates that Schumann resonance has a significant response to lightning activities of different areas all over the world. We can monitor and deduce lightning activities in "three hot spots" timely through the signals received by two orthogonal magnetic antennas of an individual ELF field station.(3) The local terminator effect in the Schumann resonances has been analyzed and discussed. Based on the OTD/LIS satellite data and the hot spot model of "activate" definite parts of global thunderstorms, according to the theoretical model of ELF radio propagation model in uniform Earth-ionosphere cavity, taking the conductivity profile of the ionosphere into account, the diurnal-seasonal variations of Schumann resonance amplitudes observed by local time has been computed. The simulation results shows distinct signatures of the terminator effect in Schumann resonance and indicate that the day-night asymmetry of the Earth-ionosphere cavity is not the primary cause for the terminator effect in Schumann resonance signals. The distribution of global lightning and the variation of SOD (source-observation distance) have been separated for further study. The results shows that lens-shaped structure of the Schumann resonance field variations is crafted primarily by the diurnal and seasonal thunderstorm drift. Meanwhile, the land-ocean distribution of lightning has also a modulation effect on the day-night asymmetry of the terminator effect. |
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