I. Introduction
Very low frequency (VLF) radio waves propagate in the "earth-ionosphere waveguide", and the propagation is relatively stable. However, the noise in the frequency band is large, and the main noise source is atmospheric noise. VLF atmospheric noise is mainly caused by lightning instantaneous discharge in the world. Because the occurrence time of lightning discharge is random, and the noise intensity is related to frequency, geographical location, season and other factors, the analysis of atmospheric noise becomes more complex.