Modeling Whistlers Generated by Gigantic Jets
Kaiti Wang1, Sung-Ming Huang2, Shu-Chun Chang2, Rue-Ron Hsu2
1Institute of Space and Plasma Sciences, National Cheng Kung University, Tainan, Taiwan
2Department of Physics, National Cheng Kung University, Tainan, Taiwan
The characteristics of whistlers generated from a gigantic jet (GJ), specifically the tree-like GJ, which possessed the strongest intensity, is modeled for the first time. Modeling is based on disturbances in the electric field, as measured by NCKU ELF/VLF station, associated with a GJ event over typhoon Lionrock. The power spectrum of GJ differs from that of common cloud-to-ground lightning; therefore, this study also investigates differences between GJ-generated signals and common lightning-generated whistlers. Detectability is evaluated by considering the absorption of amplitudes resulted from particle collisions associated with the propagation of generated waves. Our results show that the waves are subject to greater attenuation as the frequency increases; however, a reversal occurs at lower frequencies of a few hundred Hz. The calculated waveforms show that the emissions generated by the GJs are preceded by small fluctuations at high frequencies generated by the initiating lightning. Overall, the amplitudes increase with the passage of time; however, they are more randomly-distributed for whistlers generated from common lightning. The amplitudes decrease again when lower-frequency components below a few hundred Hz arrive. The amplitudes drop to the order at least above 1 mV/m as the waves propagate in the ionosphere, which puts them within a range detectable by the instruments on most satellites. There will be also two whistlers shown on the f-t spectrogram with the early one from initiating lightning, and the later one from the gigantic jet itself, strongest below about 3 kHz. Based on the locations of GJ events observed by ISUAL (Imager of Sprites and Upper Atmospheric Lightning), regions of the western and southeastern Pacific Ocean, as well as North Africa are the most likely locations to detect these emissions. We would also show that this model can reproduce features of the knee whistlers as often observed at high latitudes and also the whistlers generated by common lightning.