Isual Publications 2014

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2011 11 吳宜軒(2011),‘利用GPS觀測探討磁暴期間電離層電子濃度變化’,國立成功大學物理研究所(碩士論文)

由於電子濃度的擾動會對訊號產生干擾。為了研究磁暴期間電離層電子濃度每天的變化,此篇研究以統計方法分析電離層電子濃度(TEC)在中緯度與低緯度的擾動,使用的資料是在2000~2009年之間東亞與美洲經度的GIM資料,由GPS地面接受站資料處理所得。 
  為了比較磁暴對電離層影響與電離層每天的日變化影響,使用了Z-test方法分別分析中磁暴與強磁暴,並把磁暴依照磁暴發生當地時間分成四類分開比較。 
  為了改善分析方法的誤差而發展出不同的分析流程。研究發現,美洲地區對磁暴的反應相較亞洲地區明顯。在白天發生的磁暴之後立即發生電子濃度異常增加的事件;而電子濃度異常減少的事件在磁暴發生之後的12小時內幾乎沒有,12小時之後才開始出現,約24小時左右是最大值,並且可以持續三天左右都還有零星的事件發生。 

This paper presents statistical study of the ionospheric total electron content (TEC) disturbances at mid- and low-latitude ionosphere during magnetic storms by using GPS receiver networks along East-Asia and America longitudes during ten-year period of 2000-2009. 

To investigate the significance of the magnetic storm effect in comparison with the day-to-day ionospheric variability, we perform the z-test analysis for moderate, and intense magnetic storm events. The storm events are further categorized into four onset local time periods to study the local time effect. Meanwhile, the study performed in the two longitude sectors provides further information for longitudinal effect of the ionospheric storm effects. 

2011 12 何紹民(2011),‘鹿林觀測站極低頻磁場訊號特徵與台灣地震活動之關係’,國立成功大學太空天文與電漿科學研究所(碩士論文)

我們分析成功大學ISUAL團隊在鹿林前山(東經120.87° , 北緯23.46°, 海拔2862公尺)建置的極低頻磁場量測系統所紀錄之磁場資料,我們利用此量測系統的兩根分別平行於地磁南北向(H)與東西向(D)之天線所偵測之資料,研究2009年至2010年頻率低於10赫以下訊號與地震之相關性。 
在本研究中,我們使用極化參數分析法的計算程式,用以分析天線所偵測到之兩方向極低頻磁場訊號的物理參數,可用來了解訊號的極化特性以及其偏振軸與南北向天線的夾角。目前我們分析並比較2009年10月至2010年09月共為期一年的各項資料,並使用傅立葉頻譜與極化參數,來計算極低頻磁場訊號之相關參數並與地磁及地震活動比較。在地震資料方面則是依震源發生處之斷層性質將其分為三區(台灣島區、琉球海溝區和呂宋島弧區),並比較與極低頻磁場分析結果的相關性。另外,並與地磁指數比較,排除訊號是由地磁擾動或磁暴所造成的可能性。結果顯示極低頻磁場異常訊號和地震活動的相關性較高,特別是台灣島上斷層所發生的地震,其在地震發生前較其他兩區有更高度的相關性,而與地磁擾動或磁暴的發生並無直接且顯著的關聯性。 

We analyze the magnetic field data recorded by ELF antennas operated by ISUAL project of NCKU at Lulin Observatory in Taiwan (120.87°E , 23.46°N, 2862m). The data adopted in this analysis is from year of 2009 to 2010 with wave signals below 10Hz primarily investigated. The signals are measured in both H (north-south) and D (east-west) components. 
In this study, we focus on developing computing codes to derive parameters associated with polarizations from these two observed components. The ellipticity, orientation, and sense of polarization can then be calculated. We use Fourier and polarization analysis to analyze ELF magnetic field data and then compare these ELF results with geomagnetic and seismic data from 2009 to 2010. The seismic events are categorized in three different regions( Taiwan Island, Okinawa trough, Luzon Arc), based on the fault feature of the epicenters, and the association with the analyzed ELF event parameters are studied. In addition, the geomagnetic index are also compared and the ELF emissions are found to have little possibility to be deduced from geomagnetic activities or storms. The results suggest that the observed ELF disturbances are more sensitive to seismic events before main shock, especially for events occurred on Taiwan Island compared with the other two regions. These results also show us the ELF signals do not relate with geomagnetic activities and storms. 

2011 13 黃鵬宇(2011),‘2010年極低頻磁場波型量測分析應用於高空短暫發光現象之極性統計’,國立成功大學太空天文與電漿科學研究所(碩士論文)

極低頻波段的電磁場訊號量測,是一項能夠用來長期、全球性監控閃電活動的方法。但是連續紀錄下累積的龐大波形資料,除了單一事件比對,很難達成系統性統計分析。在本論文中,我們發展了一套演算法,自波型訊號中擷取與處理瞬變事件訊號,希望能夠達到資料即時處理、動態調整與高效率的目標,將得到的電磁參數建立成為極低頻瞬變事件資料庫,以進行進一步的統計分析。 
本論文中使用了成功大學鹿林山極低頻磁場量測站的觀測資料進行處理,並建立了瞬變事件資料庫,資料區間從2010年3月至2011年2月,在一整年完整的資料中,我們發現鹿林測站所記錄的瞬變事件發生率和台灣周圍的天氣變化存在高度的相關性,反而全球性的變化特徵並不明顯。這也顯示了鹿林測站的偵測能力和距離也具有相關性,這點在本論文中,透過全球閃電定位網路(WWLLN)獲得驗證。結合目前全球唯一在太空中對高空短暫發光現象(TLE) 作長期性、全球性觀測的衛星酬載─福衛二號高空大氣閃電影像儀(ISUAL)之資料,我們進一步的由瞬變事件資料庫中得到高空短暫發光現象的放電極性。 
在一年資料的比對分析中,我們發現紅色精靈的確與正閃電有高度的相關性,並且在這年的資料中辨認了四個由負閃電所觸發的紅色精靈,所得到的負極性紅色精靈比例已經高於之前地面觀測所得到的結果,值得進一步的分析。觸發淘氣精靈的正、負閃電極性比例約為45比55,但是一般閃電中正、負極性的比例約為1比10。由於淘氣精靈必須由強閃電所觸發,因此,這樣的結果可以顯示正閃電強度一般來說比負閃電來得強。在精靈暈盤的閃電極性分析中,我們發現接近2/3的事件是由負閃電所觸發,顯示出精靈暈盤與負閃電可能有關連性。

The electromagnetic field measurement at the band-pass of extremely low frequency (ELF) is the only way to monitor the long-term lightning activities globally. In addition to the single event analysis, huge amount data collected by this continuous recording system become a challenge to pick up and process the transients’ signal buried in systemmatically. In this work, we develop an efficient, adaptive and real-time algorithm to extract and process the transient events from the raw sferic data and to construct a transient event database with their electromagnetic parameters for further analysis. 
The data recorded by the NCKU Lulin ELF station between March 2010 and February 2011 is used in this work. After the ELF transient database is construced, we find that the transient occurrence rate recorded by Lulin station shows a strong association with the features of local, not global, climate. This result also hints that the detection efficiency of Lulin station is distance dependent, and it is also verified by direct comparison with the WWLLN database. 
The polarity of the TLEs are dertermined by analyzing coincident events with the ISUAL TLE database. We notice that there is a highly correlation between the sprite and the +CG as literatures reported, and 4 –CG sprites out of 107 are recognized respectively. This ratio of 4.4% is greater than the previous result of less than 1% obtained by ground observations in the past years. The ratio of –CG and +CG elves derived in this work is approximately 45:55, but the ratio of –CG and +CG of lightning is about 1:10 generally. This interesting result implies that the average intensity of +CG is probably greater than –CG owing to elves are triggered by the lightning with high peak current typically. We also find that 2/3 halos are associated with –CG strokes. Therefore, it might exist a dependency between halos and –CG lightnings.

2012 14 邱政修(2012),‘使用探空氣球進行大氣直流電場量測’,國立成功大學太空天文與電漿科學研究所(碩士論文)

大氣垂直電場在大域電路及各種高空短暫發光現象形成機制的理論模型中,扮演著重要的角色。本論文的目標在發展實驗方法,於30公里高度內中進行大氣垂直電場的實地量測,進而瞭解雲層電荷分佈與電位變化。我們設計了一套垂直電場電位計,量測已知間距兩端點的電壓差,就可以得到電場大小,並且我們也設計製作了一套平板校正電場,完成電場電位計的實驗室內校正與測試。在本論文中,我們使用探空氣球做為飛行載具,氣象探空氣球可以將1公斤級的酬載儀器送到30公里高空。我們也同時發展了輕型的航電系統,配合垂直電場電位計電位,用來量測不同高度時的垂直電場,以及環境氣象資訊。在2012年5月31日於台東所進行的飛行實驗中,探空氣球飛掠了一個發展中的對流系統,成功的取得了大氣垂直電場的數據與溫、溼度資料,經由分析,我們觀測到高空電場值大小隨著高度上升呈現指數衰減的情形,並且也觀測到雲層間電荷排列的層狀結構,以及在劇烈擾動區內,因為電荷無法有效的分離因而產生高度電中性的現象,而在對流系統附近電場值有明顯的變化。

The atmospheric electric field plays an important role in the theoretical models of the global electric circuit and the various types of the transient luminous events. In this thesis work, we develop a new experiment method to have an in-situ measurement of the vertical electric field up to the altitude of 30km to explore its spatial and temporal variation and the charge distribution. A lightweight electric field meter (EFM) is developed to measure the potential difference between two probes with a fixed distance and the electric field is derived directly by its definition. An in-lab facility with two parallel slabs to produce uniform electric field is designed carefully for the purpose of the calibration and test of the EFM. The metrological balloon is used as the flight carrier to deliver a 1kg-class payload to 30km height. We also develop compact and lightweight aviation electronics to obtain ambient metrological parameters. The results of the flight in May 31, 2012 are analyzed and reported in this thesis. The electric field meter flied over a growing convection system and measured the vertical electric field as well as ambient temperature, humidity, and magnetic field vector successfully. An exponential-decaying background electric field with altitude is observed. And we also identify several layer-distributed charges in different height, and the electric field become electrically neutral in the turbulent zone due the difficulty of the charge separation. A significantly variation of the electric field is also identified in the proximity of the convection system.

2013 15 李立柔(2013),‘由分析ISUAL紀錄的Secondary TLEs探討雷雨雲至低電離層的耦合現象’,國立成功大學物理研究所(博士論文)

高空短暫發光事件(Transient luminous events, TLEs)為發生在雷雨雲頂至電離層底部空間的大尺度發光現象,目前一般認為其發生與雷雨雲內的放電活動有關。早期的TLEs觀測大部分是從地面、飛機或太空梭上進行,然而自從福爾摩沙衛星二號其科學酬載ISUAL (Imager of Sprites and Upper Atmospheric Lightning)於2004年5月發射升空後,提供了一個由太空觀測TLEs的重要平台。在九年的觀測中,ISUAL紀錄了一些特別且含有多重TLEs的事件,其中主要有包括secondary sprites、secondary jets、secondary gigantic jets(secondary GJs)以及GJ-induced sprites。本篇論文中,由ISUAL光學資料以及極低頻電波訊號 (Ultra Low Frequency, ULF)數據,分析各類別特殊事件的產生特性,並且提出可能的引發機制。此外,我們藉由凖靜電場模擬計算,試著驗證secondary sprites及GJ-induced sprites引發機制的假設是否合理。 
分析ISUAL所記錄的sprites發現,約7%的sprite事件含有二個或多個接連發生的sprite,在先導sprite發生後,在不同空間位置又接續發生另一sprite。大部份的後續sprites相對於先導sprite有水平上的位移,這類sprite常為地面觀測所紀錄且被稱為是dancing sprites。然而只有三個follow-up sprite與preceding sprites具有垂直方向的相對位移,在此我們稱為secondary sprites。根據ISUAL及ULF的資料分析,我們推測dancing sprites及secondary sprites可能與閃電之後,在雲內繼續延伸的leader有關,雲內的leader會將電荷持續傳導至地表,形成continuing current或second stroke。Dancing sprites和secondary sprites的引發差異,可能是與雲內leader延伸方向有關。當leader的延伸方向主是朝水平方向時,較易引發dancing sprites;若是朝垂直方向延伸時,則較易引發secondary sprites。凖靜電場模擬結果顯示,當雲內有垂直方向的continuing current時,在preceding sprites以下區域的電場的確會增強。 
根據前人地面觀測指出,在sprites發生後,有時會引發secondary jets的產生,從雲頂向上放電至sprites底部 (~50公里)。ISUAL在衛星觀測上,也紀錄到類似的事件,除了secondary jets之外,還有另外一種類似secondary jets事件,其最終高度約至電離層底部 (~90公里),在此稱為secondary gigantic jets (secondary GJs)。從2004年7月至2012年5月,ISUAL總共紀錄了二十七個secondary jets,以及五個secondary GJs。根據secondary jets及secondary GJs的分析,我們認為形成secondary jets或是secondary GJs的主要原因為雲內殘留的負電荷的分佈及多寡,以及secondary jets/GJs發展區域的電離層高度(local ionosphere boundary)。因此,在secondary jets/GJs前發生的sprites,主要扮演的角色是造成區域的電離層高度降低,進而影響secondary jets/GJs向上的發展最終高度。 
由五個secondary GJs事件中,發現三個可能由secondary GJ引發的sprites (secondary GJ-induced sprites),且這三個事件彼此有相當類似的發生序列。一開始先有一個閃電引發的sprite產生,約在30-50微秒後,有一secondary GJ由雲頂噴發至電離層底部,然後在secondary GJ發生後約一微秒後,有一新產生的sprite出現在secondary GJ的邊緣,但其間並無對應到明顯閃電訊號。因此,根據ISUAL影像、光譜以及ULF的資料分析,這三個事件中新產生的sprites有可能是由secondary GJ引發的。另外ULF資料顯示,secondary GJ電流矩的峰值 (peak current moment)比一般GJ的峰值大,此特性可能是secondary GJ較一般GJ容易引發sprite的重要原因之一。而根據凖靜電場模擬計算結果可知,放電較快速的secondary GJ在70-80公里間產生的電場,會比放電較慢的一般GJ產生的電場大,此結果符合我們推測。

Transient luminous events (TLEs) are large-scale luminous emissions occurring in the region between thundercloud tops and the lower ionosphere, and are closely related to the underlying thunderstorm electric activities. TLEs observation are usually carried out on the ground, onboard the spacecrafts, or on space shuttles. ISUAL (Imager of Sprites and Upper Atmospheric Lightning) payload onboard the FORMOSAT-2 satellite is the first space-borne experiment with the long-term TLEs survey as its main mission goal, and has contributed substantially toward our understanding of these natural phenomena since it was launched in May 2004. In this thesis, some multi-TLE events, mainly events contain secondary sprites, secondary jets, secondary gigantic jets (secondary GJs) or GJ-induced sprites, are analyzed using the optical and electromagnetic ULF (Ultra Low Frequency) data. The possible generating scenario for each type of secondary TLEs is proposed according to their observable features. Furthermore, quasi-electrostatic field model calculations are carried out to validate the proposed generating scenarios for secondary sprites and GJ-induced sprites. 
From analyzing multi-sprite events, it was found that ~7% of them start with a classical sprite and then another sprite soon followed with a spatial displacement from the preceding sprite. Most of the multi-sprite events were dancing sprites with a horizontal shift between sprites. However, we also found three secondary sprites, that hadn’t been reported before and exhibit vertical displacements from preceding sprite. >From the analysis of spectral and ULF data, we propose that the successively occurring dancing sprites and the secondary sprites are related to the extending leaders of the cloud-to-ground lightning, which are often followed by a continuing current or even a second stroke. The dancing sprites may be induced by the subsequent leaders in the cloud extending mainly in the horizontal direction, while the secondary sprites may be triggered by the leaders extending primarily in the vertical direction. Through performing quasi-electrostatic field modeling with three different sets of input parameters, we have confirmed that the electric field in the region below the preceding sprites could be enhanced by the vertical-extending continuing current. 
Previous ground observations had reported that a secondary jet sometimes formed under the preceding sprite and then propagated upward from the cloud top toward the lower edge of the preceding sprite. From ISUAL observation, beside secondary jets, we find some secondary TLEs resembling secondary jets but with higher terminal altitudes (near the lower ionosphere boundary), hence these gigantic secondary jets are termed as ‘secondary gigantic jets’ (secondary GJs for short). Between July 2004 and May 2012, ISUAL recorded 27 secondary jets and 5 secondary GJs. Combining the observational features of the secondary jets/GJs, it is believed that the factors in influencing the generation of the secondary jets/GJs include the height of the local ionosphere boundary, and more importantly the abundance and the distribution of the negative charge left in the cloud. It appears that the preceding sprite mainly exert its influence on the secondary jet/GJ by perturbing the local ionosphere height. 
Three possible secondary GJ-induced sprites were recorded by ISUAL and shared a similar generating sequence. Each event began with a +CG-induced sprite, and a secondary GJ followed within ~30-50 ms. Then, 1 ms after the secondary GJ, a new sprite occurred near the GJ without a discernible, associated impulsive lightning signal. Cross-analysis of the spectral, image and electromagnetic data of these three events indicates that the new sprites were likely had been induced by the secondary GJs, and the high current moment of the secondary GJs appears to be a crucial factor for the induction of the new sprites. From the quasi-electrostatic modeling, it can be concluded that the secondary GJ, being a faster discharge, can produce a stronger electric field around 70-80 km than the typical type-I GJ.

2013 16 黃崧銘(2013),‘各類高空短暫發光現象的超低頻與極低頻至甚低頻電波特性’,國立成功大學物理研究所(博士論文)

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.

2014 17 陳彥廷,”鹿林山觀測站十赫以下極低頻輻射事件之方向角分析”,碩士論文,國立成功大學太空與電漿科學研究所,2014

分析在2009年10月至2010年9月在成功大學鹿林山觀測站所設置南北方向與東西方向兩根特殊天線所測得10Hz以下的特殊極低頻率之磁場訊號,發現訊號在4Hz至6Hz之間有著特殊的躍頻事件(Ho et al., 2013)。根據所描繪出的時頻圖,頻率變換會在一天之內發生,之後持續數天再度變換。 
為了更加瞭解躍頻事件和相關資訊,在本篇論文內以一天為單位將訊號經傅立葉轉換後得到其功率並加以分析,再利用極化分析的方法分析各個頻率的極化參數,任何波場的偏振方向特性可以利用相干矩陣的元素來描述,瞭解其極化特性以及其偏振軸與南北向的夾角,以期分辨各個訊號的性質。 
接下來分析訊號的磁場方向角,由東西向與南北向每個不同頻率資料點的結果計算出每個單獨的磁場方向角,最後再將所計算出的方向角加以平均即得到在特定時間內的平均磁場方向角。 
最後將鹿林觀測站天氣狀況資料以及大氣化學測站團隊的上下山時間與躍頻事件發生時間進行比對,討論訊號可能的來源以及改變頻率的原因。

Special magnetic emissions were found below 10Hz at Lulin ELF station by analyzing observational data from October 2009 to September 2010. The events are especially for the frequency-shift events between 4 and 6 Hz. All the frequency-shift occurred within one day, and can last for several days, according to the f-t spectrograms plotted on monthly base. (Ho et al., 2013). 
To understand the source of the phenomena of frequency-shift and the associated emissions, this thesis investigates the frequency-shift phenomena in detail based on single-day f-t spectrograms. 
The Mean’s method is adopted to define the polarized direction of the incoming signals. The polarization of any wave fields can be characterized in terms of the coherency matrix elements and then we can use the above information to estimate the trait of signal and the direction of polarization axis. At the same time, we analyze the direction of magnetic fields using FFT (Fast Fourier Transform). 
Finally, the weather condition of the Observatory as well as the nearby activity from measurement team of atmospheric chemistry are compared with the occurrences of frequency-shift events. The possible sources of these signals and reasons for frequency shifts are discussed.

2014 18 莊嘉文,”發展電場量測儀器進行地表大氣電場量測”,碩士論文,國立成功大學太空與電漿科學研究所,2014

大氣電場在大域電路中扮演著極為重要的角色,從一般的晴天天氣到劇烈天氣,大氣電場可以有高達數個數量級的變化。因此,地表的電場量測可以做為閃電危害的預警之用。近年來的研究顯示,在岩石圈-大氣-電離層的耦合系統中,電場也扮演了重要的角色,因此地表電場量測方法的改進,對於上述問題,將可提供更多觀測上的證據。 
電場量測儀其原理是藉由改變垂直電場的感測面積造成電極之感應電流變化量,以量測出電場強度。本論文的目標在於發展並自製電場量測儀,以低成本、低耗電量、小體積、不需要連接電腦、所需資源少以及容易架設做為設計目標。利用其高機動性,可望架設在偏遠地區,避免人為活動所造成的電場干擾,並逐步建構成為大範圍的即時電場監視網。 
電場強度可由電極電壓訊號以及轉速訊號計算得知,因此需以適當機械結構與電路量測上述兩種訊號,並搭配中央處理器做後端數據紀錄與分析。在儀器校正方面,使用平板校正電場,在實驗室內進行此電場量測儀器之測試與校正。 
量測訊號經分析後,其結果與理論相符合,也證明了此儀器可用於量測大氣電場。未來希望藉由實地量測,觀測到劇烈天氣下的擾動電場以及大氣電場之長期變化。

Atmospheric electric field plays an important role in the global circuit. It varies in several orders of magnitudes from the conditions of fair weather to thunderstorm or dust storm. The electric field at ground can act as an alert indicator of the possible damage caused by the severe weather. Recent studies show that the electric field also plays a significant role in the large-scale coupling between the lithosphere, atmosphere and ionosphere. Therefore, the measurement of the electric field above the ground becomes essential. In this study, we developed an instrument field mill which has the features of low cost, low power consumption, tiny size, no need to connect computers, and easy to set up, etc. With these features, we are able to construct a wide network by deploying sets of this instrument in some distant and remote sites to monitor the real-time change of the ground electric field for the studies in various research fields including the precursor of the earthquake. Finally, an outdoor experiment was conducted to show the practical measurement, and the results confirm that this field mill satisfies the requirement of the design goal.

2014 19 王睿翰,”運用希爾伯特黃轉換法探討鹿林山觀測站10赫以下極低頻輻射事件”,碩士論文,國立成功大學太空與電漿科學研究所,2014

台灣極低頻磁場量測系統是由成功大學 ISUAL 團隊在中央大學鹿林天 文台之北側所建置的,該系統可量測紀錄極低頻磁場擾動的訊號資料。在之 前的研究文獻中,曾發現快速傅立葉轉換法分析這些磁場資料的頻譜結果顯 示,頻率低於十赫茲以下的訊號時而發生特殊之躍頻事件,即訊號較強的輻 射頻率在頻譜圖上顯示由低頻變為高頻,或由高頻變為低頻。 
在本研究中,有別於傅立葉分析方法,我們使用希爾伯特黃轉換法分析 這些極低頻磁場擾動之訊號,探討在不同的時間尺度下,訊號頻率隨時間的 變化。並針對快速傅立葉轉換頻譜上顯示的躍頻事件發生的時段來探討同時 段內希爾伯特黃頻譜上的輻射頻率特徵。 
我們分析並比較 2009 年 10 月至 2010 年 09 月為期共一年的觀測資料, 發現在低於舒曼共振基頻之下,確實存在兩條強度較明顯的輻射。這些輻射 之頻率涵蓋範圍較傅立葉轉換法所呈現之輻射頻率範圍廣。以躍頻上升事件 為例,可發現多數事件發生後此輻射頻率之標準差變小的現象;反之,對躍 頻下降事件而言,大多數的事件發生後,此輻射頻率之標準差變大。這些現 象無論在地磁東西向或南北向上的分析結果都可發現,但發生在地磁東西向 的事件數多一些。總之,由希爾伯特黃轉換法分析訊號資料的結果來看,十 赫茲以下也有輻射頻率變遷的現象存在。

The ELF magnetic field measurement system in Taiwan was setup by the ISUAL team of National Cheng Kung University at Lulin Observatory managed by National Central University. This system measures magnetic fields at the band of extreme low frequency (ELF). In previous studies, through analyzing the signals using Fast Fourier Transformation (FFT), special emission events with frequency shifts below 10Hz were found to occur sometimes on the frequency-time spectrograms. That is to say, the frequency of emissions with higher intensities can vary from lower-frequency to higher-frequency, or from higher-frequency to lower-frequency. 

In this study, other than the method of Fourier analysis, we adopt the Hilbert-Huang Transform (HHT) as the tool to analyze these measured ELF magnetic field signals. The variations of emission frequencies are explored based on data with different time-scales. The characteristics of emission frequencies at frequency-time spectrograms acquired from HHT are also investigated for the time periods defined by the frequency-shift events shown on the time-frequency spectrogram based on FFT. 

We analyzed one-year-long data measured from October 2009 to September 2010 and find there do exist two emission bands with enhanced intensities. These emission-bands cover wider ranges in frequencies compared with those derived from FFT. For most frequency-up events, the standard deviations in emission frequencies are found to decrease at the time after their frequencies shift up. Conversely, for most frequency-down events, the standard deviations become larger at the time after their frequencies shift down. in frequency decreased case. The above features occurred no matter in geomagnetic east-west or north-south directions, but more often in east-west direction. Therefore, from the analysis through HHT, these measured signals are also found to have variations in emission frequencies below 10Hz.

2015 20 陳毅倍,”閃電與高空短暫發光現象之放電極性全球分佈統計”,碩士論文,國立成功大學太空與電漿科學研究所,2015

高空短暫發光現象由雲間閃電或雲對地閃電所引發,與閃電極性的關係在過去幾十年間已有相當的研究。例如,超過99%的紅色精靈由正極性雲對地閃電所觸發,而淘氣精靈目前仍沒定論。全球性閃電與高空短暫發光現象的放電極性統計可以讓我們了解閃電極性對於發生率的影響。由於目前並沒有單一資料來源可以進行這樣的研究,我們使用安裝於中央大學鹿林山天文台的極低頻波段磁場量測資料結合全球閃電定位網路資料庫(WWLLN)和ISUAL任務的高空短暫發光現象事件資料庫,進行閃電與高空短暫發光現象之放電極性全球分佈統計。 
在本研究中,共分析了2010 年3 月至2011 年2 月區間中的閃電與TLEs 
事件,藉由本論文所發展的演算法快速而正確的自極低頻波段磁場中解析出 
放電極性。分析結果顯示在全球分佈上,距離台灣3000 公里內範圍內,閃電極性偵測率可以達到70%以上,在此範圍內,負閃電的數量約為全部閃電的90%。超過3000 公里範圍外,閃電的極性偵測率隨距離遞減至10%左右,而負閃電的比例下降至50%左右,這樣的結果也顯示了正閃電的平均能量較負閃電高,這也為最近的研究結果提供了佐證。 
高空短電發光現象的統計結果顯示,資料期間內所觀測的紅色精靈大於95%都是由正極性閃電所引發,可判斷極性的48 個紅色精靈中僅有一個事件為負閃電相關,這和之前的研究結果相符。令人驚訝的是大於95%的淘氣精 
靈都是由負極性閃電所引發,這和先前的研究不同,主要的原因是因為ISUAL 近年根據電波觀測結果做了時間修正,把儀器觸發時間的精確度由25毫秒修正到3 毫秒以內,使得誤判的比例大幅降低。在閃電與高空短暫現象的季節分佈中,可以觀察到TLEs 現象隨著閃電數量增加而增加,兩者之間的發生率存在著正相關,但是閃電的極性沒有顯著的季節性變化,這樣的結果也顯示高空短暫發光現象的分佈主要受閃電的密度與能量影響,放電極性的空間與時間差異極小,影響不大。

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. 

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Wednesday the 17th. ISUAL. All rights reserved.