高空短暫發光現象在雲層至電離層間的電荷搬運扮演極為重要的角色，通常事件發生的時間長度從小於一毫秒到數百毫秒，由於過程極短，所以使用傳統時頻分析方法對於瞬間過程的細節研究相對較少。希爾伯特·黃方法在近年來已廣泛用於在其他領域的時變頻譜分析，希爾伯特·黃轉換藉由對訊號相位的微分，獲得其瞬時頻率的變化，相較於一般傳統傅立葉頻譜分析，無法針對短時間內，快速的訊號變化給予頻率上良好的定義。本研究對鹿林觀測站極低頻磁場量測系統所記錄到的高空短暫發光現象，進行瞬時頻譜的分析。透過分析2012年到2014年間台灣光學觀測網路所觀測到的巨大噴流在希爾伯特頻譜上的特徵，發現了72%的巨大噴流有早期訊號的存在。經由模擬訊號的分析和光學影像上的比對，證實這些早期訊號是來自於雲層間的放電現象，這也為巨大噴流形成機制的理論模型提供了觀測上的佐證。我們也統計了2012年到2014年間，台灣光學觀測網路所觀測到其他類型高空短暫發光現象與早期訊號的關係，結果顯示其他類型的高空短暫發光現象只有不到30%的事件可以找到疑似早期訊號。因此除了巨大噴流外，其他類型的高空短暫發光現象幾乎無法確認早期訊號的存在，這也間接證實其他類型高空短暫發光現象的觸發機制應該不是來自於雲層間的放電現象。

The conventional Fourier analysis on the sferics at ELF and VLF frequency bands have been explored for decades. Several phenomena, e.g. infrasound and Schumann resonance, have been well studied by the Fourier spectrogram comprehensively. But the core computation of the Fourier analysis is an integration over a window of a specific time length, therefore, the temporal and frequency resolutions are limited not only by the sampling rate but also the length of the integration window. The instantaneous frequency can’t be obtained through this conventional approach precisely.
The Hilbert-Huang transform (HHT) is introduced in this study to analyze the sferics of transient luminous events (TLEs) recorded at the Lulin observatory. The Hilbert–Huang transform decomposes a signal into so-called intrinsic mode functions (IMFs), and derive instantaneous spectrum via the Hilbert transform. The gigantic jets registered by the Taiwan Ground Optical Observation Networks were used in this study and an early signal of frequency-change before the phase of the leading jet is revealed surprisingly. Furthermore this early signal cannot be identified in the conventional Fourier spectrogram. By a simple simulation, this frequency variation is confirmed as a nature of the discharge, not an alias or a false signal generated by the analysis method. The results of the statistics on the TLEs observed from 2012 to 2014 show that this early signal can be discriminated in more than 70% of the gigantic jets, but only less than 30% of other types of the TLEs are able to identify this feature in the HHT spectrogram. This early signal as well as the polarities of the observed gigantic jets strongly agree with the theoretical model proposed by Krehbiel et al. [2008].

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