Power spectrum density analysis of intra-cloud lightning discharge components from electric field recordings in Poland

The aim of this paper was to show usefulness of Power Spectrum Density analysis for identification of different components of IC lightning. We have presented some exemplary pulse train electric field recordings characterized for the specific time development of intra-cloud lightning discharges observed during summer thunderstorms in Poland. These data were gathered by two detection stations working independently in Warsaw and Rzeszow. Both stations are based on triggered electric field acquisition in the ELF-MF frequency range. Such recorded electric field courses of intra-cloud lightning were then analyzed both in the time domain and in the time-frequency domain with application of the short-time Fourier transform (STFT). On the basis of the used STFT procedure the Power Spectrum Density (PSD) spectrograms were computed. This whole computer procedure was performed by using the Matlab platform. Due to that we were able to distinguish some different intra-cloud lightning components as the preliminary breakdown stages including sequences of initial pulse bursts, K- and J-type electric field changes and sometimes even a trace of the Narrow Bipolar Pulses (NBP's). We have obtained different shape/pattern of the particular PSD stripes related/corresponding to the considered components of intra-cloud lightning. Summarizing of our PSD analysis of the recorded intra-cloud lightning incidents we have noticed that the PSD stripe structure of the initial breakdown processes for in-cloud and cloud-to-ground lightning is similar. But, the characteristic and different shape of the PSD stripe was obtained for the K-type electric field changes occurring during intra-cloud lightning. For these cases the PSD stripes were of the same height and width as for the return stroke incidents from the cloud-to-ground flash, but their intensity for the frequencies above 500 Hz was much lower. On the other hand, the PSD stripe obtained for the NBP was very often the strongest one and therefore it was hard to be properly distinguished in contrast to the return stroke stage from the cloud-to-ground flash.