| 摘要: | 精準測知當下電離層天氣(ionospheric weather)之活動情形，對於預警電離層天氣對電磁波通訊和導航系統的干擾極其重要。電離層電漿密度結構能對電磁波產生影響，而此影響亦能用於推估全球定位系統(Global Positioning System, GPS)衛星與接收機之間的全電子含量(total electron content, TEC)。鑒於此，本論文運用地基(ground-based)與天基(space-based) GPS觀測全球電離層電漿密度變化，對區域和全球性的電離層天氣活動進行深入研究。
地基GPS接收機廣布全球大陸，能高效且經濟地提供高時間解析度之TEC觀測。而GPS掩星(radio occultation)系統如福爾摩沙衛星三號(FORMOSAT-3/COSMIC)能提供全球均勻分佈之三維電離層電子密度數據，填補地基GPS接收機在海洋、沙漠與極區的觀測空缺。此外，由於電磁層資料通常稀疏(sparse)且複雜，簡單的資料分析方法與傳統的簡諧(harmonic)和穩態(stationary)假設不足以從觀測數據呈現現象的完整面貌。因此本論文使用較進階的方法，如非線性(nonlinear)與非穩態(non-stationary)數學方法、統計工具、資料同化(data assimilation)技術和物理模式，對GPS觀測之電離層天氣現象進行分析。
本論文第一章簡介地球電離層與電離層天氣。第二章簡介地基與天基之GPS觀測資料。第三至第六章以四個太空天氣案例對GPS資料及其分析方法的使用進行深入探討。第三章使用非線性非穩態的時頻分析方法，希爾伯特黃轉換(Hilbert-Huang transform, HHT)，分析地基GPS TEC後發現全日蝕能於地球電離層中產生船艏波。第四與第五章分別研究磁暴對低緯與中緯電漿密度不規則體之影響。研究結果有助於臆測磁暴後不規則體之產生與消失。第五章率先使用非穩態的“小波共變異數(wavelet-based covariance)”建立區域性的全電子含量圖(TEC map)，此圖有助於研究極光橢圓區附近小尺度的電漿密度結構。第六章建立“模式中性風偏差修正程序(model neutral wind bias correction scheme)”以精進“電離層電漿層電動模式(Ionosphere Plasmasphere Electrodynamics model, IPE)”模擬中緯電離層電子濃度之能力。經由比較IPE模式之模擬與福衛三號掩星之觀測結果後發現，“南半球中緯夏季夜間電子濃度異常/威德海異常(Southern Hemisphere Midlatitude Summer Nighttime Anomaly / Weddell Sea Anomaly, MSNA/WSA)”之東向漂移現象主要由中性風沿地球磁場分量所控制。此四項研究結果顯示，藉由GPS能監測區域或全球性快速改變之電離層天氣。其物理機制將於正文深入驗證與討論。
;Precisely knowing the current state of the rapidly changing ionospheric weather is important for warning about its impact on modern telecommunication and navigation systems. The effect of the ionosphere on radio waves transmitting from a satellite to a receiver can be used to estimate the plasma density along a signal path with measurements of the modulations on carrier phases and code pseudoranges recorded by dual-frequency receivers. Therefore, this thesis presents applications of global measurements of electron density from the ground- and space-based GPS observational systems to the in-depth investigation of the regional and global ionospheric weather events.
The ground-based GPS receivers, which are distributed widely over the world’s continents, provide us with an efficient and economic way to monitor the total electron contents (TECs, 1 TEC unit (TECU)=1016 el/m2) with high temporal resolutions. While the radio occultation (RO) observational systems, such as FORMOSAT-3/COSMIC (F3/C), measures the three-dimensional ionospheric electron density globally, including the oceans, deserts, and polar regions, where ground-based observatories are scarce. Since the ionospheric weather-generated signals measured by observational instruments are usually sparse and complicated (nonlinear and non-stationary), simple data process procedures and traditional harmonic and stationary assumptions may not be sufficient to expose the full aspects of the weather phenomena. Therefore, the objective of this thesis is to demonstrate how the analysis of GPS data can be aided with more advanced methods, such as statistical tools, nonlinear and non-stationary mathematical methods, data assimilation techniques, and a physical-based model.
After generally introducing the Earth’s ionosphere and ionospheric weather (Chapter 1) as well as the ground- and space-based GPS observational systems (Chapter 2), four events are investigated in detail. Chapter 3 shows the solid evidence of the eclipse-triggered bow wave with the help of the nonlinear and non-stationary time frequency data analysis method, the Hilbert-Huang transform (HHT). Chapters 4 and 5 are studies of ionospheric plasma density irregularities at low latitude and midlatitude, respectively, during geomagnetic storms. The results can help us to hypothesize the possible behavior of irregularities after storm onsets. In Chapter 5, the TEC maps are constructed by using the non-stationary wavelet-based covariance to study the finer-scale TEC structures near the auroral oval. Chapter 6 proposes a model neutral wind bias correction scheme to improve the electron density of the global physics-based Ionosphere Plasmasphere Electrodynamics (IPE) model at midlatitude. The agreement between the F3/C observations and the model simulations reveals that the eastward movement of the Southern Hemisphere Midlatitude Summer Nighttime Anomaly (southern MSNA) / Weddell Sea Anomaly (WSA) in the local time coordinate is primarily caused by the field-aligned projection of thermospheric neutral winds. The four studies shown in this thesis reveal that the GPS observational systems are capable of recording fully the features of the fast changing ionospheric weather on both regional and global scales. The physical mechanisms behind the ionospheric weather features are further examined and discussed. |
