摘要Transient Luminous Events (TLEs) are triggered by inter-cloud lightning (IC) or cloud to ground (CG) lightning. The polarity of lightning and TLEs have been studied for decades. For example, more than 99% of sprites are initiated by +CG lightning, but the polarity of elve-associated lightning is still unclear now. We can explore the connection between TLEs occurrence and lightning polarity by investigating the global distribution of the discharge polarity. Since there is no single-source data providing all necessary information for this study, multiple sources including the magnetic field measurements at extremely low frequency (ELF) band-pass, the database of the World Wild Lightning Location Network (WWLLN) and the ISUAL TLEs list are used to explore the discharge polarities of lightning and TLEs.
The polarities of the lightning and TLE events from Mar. 2010 to Feb. 2011 are investigated by an efficient algorithm developed in this study. The results show that the successful detection of lightning polarity can be up to 70% within 3000 km from Taiwan. In this region, -CG lightning accounts for approximately 90% of the total polarity-resolved events. Moreover, the polarity detection rate decays along the distance, as well as the ratio of –CG lightning also drops to about 50%. This result implies that +CG discharge is averagely more energetic than –CG events.
The in-depth analysis exhibits that the observed sprite in this period are dominantly triggered by +CG events (95%), only one -CG sprite out of 48 is confirmed and this result is comparable with previous findings. It is surprisingly founded that elves are majorly initiated by –CG lightning (95%), and this result is new and confident because the accuracy of ISUAL onboard trigger time is improved by other ELF study, and the uncertainty of ISUAL event trigger time shrinks from 25 milliseconds to 3 milliseconds. In addition, it is revealed that the density of TLE is proportional to lightning occurrence in the seasonal distribution of lightning and TLEs, but the polarity of lightning seems no significant seasonal variation is observed. This result also suggests that the distribution of TLEs is mainly governed by lightning density and energy, and less correlated with polarity.
|監督員||Bing-Chih Chen (Supervisor)|