The early ELF signals of the gigantic jets captured by the Taiwan ground observation network

Abstract:

The in-cloud ignition process of gigantic jets and blue jets receives attentions and discussions in the past years. The polarity and the position of their breakdown were proposed by Krehbiel et al. [2008] but no concrete observational evidence to support it directly. ELF spectrogram is a good tool to explore the electric activities, but traditional spectrograms are generated by a Fourier transform which obtain the frequency information through an integration operation. However the integration greatly limits the lowest frequency revealed by spectrogram and buries the important transient features. In this study, we applied a new but widely-used method, the Hilbert-Huang transform (HHT), to explore the spectrogram. Instead of the integration, HHT obtains the frequency information by differentiating on the phase angle, and become a powerful tool to reveal the fast frequency variation associated with transient luminous events. More than 100 transient luminous events including 25 gigantic jets observed by Taiwan ground optical observation network were analyzed. The results indicate that approximately 70% of gigantic jets can identify a rapid frequency variation in the interval of 300-600 milliseconds before main surge discharge, and this early feature can not find a clear corresponding amplitude variation in its sferic. Since this early signal can not be identified from the traditional Fourier spectrogram, but clear in-cloud lightning was registered correspondingly by the ground optical observation. In contrast to gigantic jets, this feature of early frequency change can be seen only in less than 30% of sprites and elves. These observational evidences are able to provide new constraints on the early discharge process of gigantic jets in clouds.