Observation And Experimental Study On The Propagation Characteristics Of Long Distance Lightning Magnetic Field

The detection and location of long-range lightning electromagnetic fields is of great significance for the theoretical study of the propagation characteristics of lightning electromagnetic waves along the ground surface and the inversion of lightning discharge parameters.Based on the theory of existing magnetic antenna design,this paper develops a high-gain lightning electromagnetic field detection system.After the application of the system in Nanjing network,it realizes long-range lightning electromagnetic field observation and location.Using the summer observation data of the Nanjing station and the Duke University station,we analyzed and experimentally studied the correlation between the lightning electromagnetic field characteristics at different distances and the relationship between the intensity of lightning current and the magnetic field amplitude.The main work and results of this paper are as follows:(1)The cross-correlation method is used to calculate the time difference between waveforms of different stations,which avoids the situation where the peak cannot be determined when there are many pulses.At the same time,the linear least squares solution is used as the initial value of the iteration,which makes the iterative process converge faster and better,further improving the accuracy of the positioning result.On the basis of multi-station positioning,the single-station positioning method was designed to solve the problem of angle correction and evaluate the error of single station positioning.(2)By using the wavelet transform to analyze the ground flash and NBE magnetic field waveforms,the time-frequency characteristics of the lightning electromagnetic field signals at different distances are discussed.Within 300 km,as the distance increases,the spectrum and energy distribution of the ground flash waveform gradually decrease;at 400 km and above,the recoil portions of the sky wave and the ground wave overlap,resulting in a widening of the recoil waveform.The frequency band is reduced and the relative energy is increased.The energy distribution frequency band of NBE is generally higher than the ground flash,and the waveform of the recoil part is narrower and the energy is relatively concentrated.When the distance reaches 400 km,the waveform overlap between the sky waves of NBE will cause the first and second sky waves.The magnitude and relative relationship of energy changes.(3)By calculating the amplitude of the lightning field at a distance of 200-1000 km from the source point,the attenuation relationship of the magnetic field strength with distance is obtained by fitting.According to the above attenuation law,the magnetic field data is normalized to 650 km,and the distribution of the intensity of lightning current with the ground wave amplitude is obtained,which approximates the linear relationship,and the fitting effect is better after considering the influence of the curvature of the earth.The normalization of a sky wave uses the "path method" based on the propagation path and the "ratio method" based on the ratio of one sky wave to ground wave.There is no significant difference in the data distribution trend obtained by the two methods,but the "ratio method" considers the influence of the ionospheric characteristics on the sky-wave signal,which is more suitable for the actual situation,the data distribution is relatively convergent,and the fitting effect is more excellent.