Time-frequency features of lightning-generated electric fields appliying the Local Polynomial Fourier Transform (LPFT)

In the last two decades, various lightning events have been analyzed using mathematical and signal processing techniques. In this paper, electric field waveforms radiated by negative first and subsequent strokes were analyzed using the Local Polynomial Fourier Transform (LPFT). The analysis provided by LPFT shows the instantaneous frequency (IF) of the signals and its first derivative. The electric field signatures came from a thunderstorm occurred in Bogotá, Colombia. In addition, the return stroke energy concentration is presented in the form of the Local Polinomial Periodogram (LPP). The LPP shows that the return stroke spectrum is distributed in a frequency range with the peak value spread in a specific part of this range. Form the time-frequency analysis provided by LPFT, the energy radiated by negative first return strokes remains in the average frequency range from low frequency (lower than 1 kHz) up to 36 kHz, while the energy radiated by subsequent strokes ranges between 0.5-42 kHz. In addition, for the initial stage and the overshoot region of the transient pulse, the first return stroke signatures present a frequency range slightly higher than those of the subsequent return strokes.