The Broadband Observation And Radiation Field Spectra Analysis Of Lightning Discharges

This paper bases on the method of lightning broadband observation and its spectra analysis, a new-type lightning broadband RF(radio frequency) observation system is designed.The broadband electromagnetic radiation is measured by a biconical antenna (HFBA9122+BBVU9135) with a bandwidth from 0.1~500MHz. The output is received by a high speed data acquisition card (NI-5122) and a Real-Time Spectrum Analyzer (RSA3408A). Another acquisition card (NI-5105) is used to record dE/dt data and the magnitude of dE/dt is used as a trigger for the whole system.During the summer of 2007 and 2008, we conducted broadband radio frequency(RF) observations of lightning at"Guangzhou Field Experiment Site for Lightning Research and Testing"and preliminary data was obtained. The results indicate that the spectral amplitude of all the three kinds of lightning discharges decrease in different rate with the frequency increasing. However, the spectrum of the NBEs show a higher radiation energy over the entire frequency interval, especially above 10MHz.A further study about CPT(chaotic pulse train) event is presented in the paper. CPT is a pulse train of irregular pulse characteristics and erratic amplitudes which are observed in lightning flash. It is observed to be strong sources of HF radiation in Negative Cloud-to-ground lightning, and its average duration in our data is 470.2us. In our study, more than 46% of the total subsequent strokes recorded are preceded by CPTs, which suggests that these chaotic pulse trains may be a ubiquitous feature precede subsequent stroke. Although the physical mechanism of CPT can not be determined based on our data set alone, we believe this unique phenomena may caused by preliminary breakdown pulses that take place inside cloud. Since these complex processes were difficult to be identified by traditional observation, they are mentioned only sporadically in the literature.Our analysis has already shown the advantages of using a broadband system in addition to traditional observation. Since it is much more sensitive to higher-frequency events, the data may bring us some important information and will be very useful for studying the physical mechanism of lightning discharges.