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Knowledge of expected parameters of natural extremely low frequency (ELF) radio signal arriving from the global lightning activity to observatory is necessary when one looks for the signals originating from other sources, in particular, for the electromagnetic precursors of a future earthquake shock. The radio waves of seismic and cosmic origin might be expected in the bands of ultra-low (ULF, 0.01-3 Hz) and ELF (3 Hz-3 kHz), i.e., at frequencies where natural electromagnetic radiation exists in the Earth-ionosphere cavity. Radiation from the magnetosphere or from underground electric activity provides additional signals to be discriminated from the ordinary lightning induced pattern. We construct the model variations of natural ELF radio signal on the daily and monthly scales. The maps of global distribution of lightning flashes are used as initial data. These were recorded by the OTD satellite (optical transient detector) and by LIS (lightning image sensor). The model developed provides expected variations of the ELF power on the diurnal and seasonal scales thus describing typical behavior of Schumann resonance background.