[M-IS13] Atmospheric electricity: Application of technology for reducing disaster risks

[MIS13-P02] Spectrogram analysis of electromagnetic signal from winter lightning cloud over Kanazawa seaside observed using lightning resilient drone

*Fumihiko Ishiyama¹, Atsushi Nagao¹, Toshihisa Masuda¹, Masao Maruyama¹ (1.Nippon Telegraph and Telephone Corporation)

We have been working on the electromagnetic observation of winter lightning cloud over Japan sea around Kanazawa area since 2021. We fly our lightning resilient drone over the sea and extend 200m long metallic wire from the drone. The wire acts as the probe for the observation. We observed several electromagnetic pulses with 10Hz sampling during the observation from Dec/2021 to Feb/2022. As the detailed information of the pulses was not obtained with the observation, we tried detailed observation with various sampling frequencies during the observation from Dec/2022 to Feb/2023. Through the observation, we obtained two electromagnetic time series. The sampling frequencies of the two are 1kHz and 1MHz. We analyzed the time series with our own method of nonlinear time-frequency analysis, plotted their spectrogram, and found repetitive pulse series structure. One has 200Hz frequency with around 10 cycles of oscillation, and the other has 40kHz frequency with around half cycles of oscillation. Detecting half cycles of oscillation becomes possible with our own method of analysis, which is a unification of various linear methods such as Fourier analysis, linear predictive coding, and singular value decomposition. As repetitive pulse series were found in two frequency regions, this kind of pulse series will be found in other frequency regions. This implies a fractal like structure of the pulse series, and this will be related to growing process of lightning cloud.