ULF and ELF/VLF Electromagnetic Signatures of Transient Luminous Events

  • 崧銘 黃

學生論文: Doctoral Thesis


In the past decade, transient luminous events (TLEs) have been the main research focus of the TLE group at the Physics Department, NCKU. Since 2001, we have routinely carried out ground TLE campaigns in Taiwan and have performed a global survey of TLEs using the ISUAL payload onboard the FORMOSAT-2 satellite since 2004. Till now, most of TLE research efforts have mainly concentrated on analyzing the large number of the ground and the ISUAL recorded TLEs using the optical and the spectral data. The occurrence of TLEs, including sprite, elve, halo, blue jet and gigantic jet, is known to be closely linked to the electric discharges in thunderclouds. However, the optical and spectral analyses provide little insight into the characteristics of the electric discharge processes that induce the TLEs. Because the lightning discharges are known to radiate the bulk of electromagnetic energy between the ultra low frequency (ULF) and the very low frequency (VLF) bands, the sferics emitted by lightning in these wavelengths can propagate in the Earth-ionosphere waveguide over a long distance with low attenuation. Therefore, in order to facilitate a better understanding of the electric discharges behind the TLE phenomena and to expand our observational capability, an ULF and an ELF/VLF radio recording systems have been installed in low electromagnetic noise sites, which locate at the Lulin Observatory and the Cingcao Elementary School in Tainan City, respectively.
In 2009, we re-built and re-calibrated the previous existing Lulin ULF station and expand the detection frequency range of the system to better monitor the sferics from the lightning return strokes and the continuing current. Shortly, a second Cingcao ELF/VLF recording system was added in late 2009 to obtain a higher time resolution sferic data for resolving the electric processes in TLE-associated discharges. With both systems, we have the capability to monitor the sferics emitted by the electric discharges that produced the observed TLEs and to infer their electromagnetic signatures. The Lulin ULF sferics recorded before 2009 suffered from phase distortions that imparted by a hardware signal modulator, which was used to filter out the usually intense 60 Hz power grid noise. In our in laboratory testing, it was found that the signal modulator incurred ~9 milliseconds of time delay on the sferics. In order to render the hardware notch-filtered sferic data being useful again, a simple signal reconstruction method was used to re-process the notch-filtered sferics. After the reconstruction, the sferics were found to contain less ringing, regain some of their lost amplitude, and more importantly have the correct event time. The fairness and reliability of the signal reconstruction method were checked using a few laboratory-generated waveforms and the results indicate that method is quite robust. With the reconstructed sprite-associated sferics, we also found a tight linear correlation between the charge moment change of the sprite-inducing CGs and the brightness of sprites. Furthermore, the threshold of the CG discharges needed to initiate sprites is also inferred and the result is generally in agreement with the accepted value.
With the renovated Lulin ULF and the newly installed Cingcao ELF/VLF sferic recording systems operate concurrently with the ground TLE observation campaigns and the ISUAL experiment onboard the FOMOSAT-2 satellite, valuable TLE-associated data are collected. The first notable opportunity was the observation of typhoon Lionrock on 31August 2010. About one hundred TLEs were observed to occur over this typhoon, among them fourteen negative gigantic jets were found to have clearly analyzable optical, ULF, and ELF/VLF data. For each GJ sferic, the signals associate with the observed optical processes, including the initiating lightning, the leading jet, the fully-developed jet (the surge current), and the trailing jet (the continuing current) were found. Furthermore, quantitative parameters of the initiating lightning, the surge current and the continuing current were inferred. These physical parameters are found to be closely linked to the morphologies, including the “tree-like”, the “carrot-like” and a newly found hybrid form called the “tree-carrot-like, of the negative gigantic jets.
Finally, to obtain the electromagnetic signatures of the TLE-associated discharges, the sferics associate with various groups of TLEs are analyzed in detail. In the global survey of TLEs, ISUAL has recorded more than ten thousand events since late 2009. From analyzing the sferics and the ISUAL spectrophotometer/array photometer data associate with pure sprites, the percentage of sprites with a sprite current is found to twice of the previous reported value [Cummer et al. 2003]. We also discovered the sferics associate with blue jets for the first time. The impulsive characteristics of the blue jet-associated sferics indicate that they may be closely linked to the narrow bipolar events (NBEs). In depth analyses, the detectability, the polarity distribution, and the characteristics of waveforms are all studied. The sferic detectability range for each group of TLEs is quite similar; the 90% sferic detectable range is 4-6 Mm typically. Among the TLE groups, only the sferics associate with halo have the farthest detectable range, since they tend to have larger amplitude. The results for the TLE polarity distribution are quite surprising. The elves are dominated by negative discharges (96%). In the well-organized Table 5 3, the negative event ratio increases monotonically across the TLE groups from 0% for the sprites to 96% for the elves. Furthermore, the polarity distribution of the causative discharges was also found to be related to their occurrence ratios over the land, the oceanic and the coastal areas. Finally, the average sferic waveforms for various TLE groups in the same 1 Mm bin are computed. The rise and decay time as well as the spectra of the average sferics are analyzed. Except for pure sprites, other group of TLEs have a similar average waveform. Only the averaged sferics associate with sprite have the longer signal duration and the energy concentrates mainly below 200 Hz.
監督員Rue-Ron Hsu (Supervisor), Bing-Chih Chen (Supervisor) & Han-Tzong Su (Supervisor)