| 摘要: | 地球高層大氣之電離層和熱氣層，在不同物理和化學機制相互作用影響下，使得其中之電子濃度隨著經緯度、高度、時間、季節、太陽活動，而呈現相當複雜的變化。這些變化肇因於太陽輻射改變、電離層與熱氣層耦合作用。本論文研究電離層電漿與熱氣層中性風之關聯，其中包括有：(1)南半球中高緯度之威德海異常(Weddell Sea Anomaly, WSA)與北半球中緯度之西伯利亞和雅庫茨克異常(Siberia-Yakutsk Anomaly, SYA)，此二者特徵為夏季夜間電離層電子濃度比其白天的大。(2)低緯度電漿匱乏灣(Plasma Depletion Bays, PDB)，其為五月前後期間赤道電離層於北大西洋、印度洋、東南亞三個區域，出現夏半球夜間大氣輝光與電子濃度北匱乏灣之現象。本研究利用「福爾摩沙衛星三號」(福衛三號)觀測電離層三維結構，詳細探討威德海異常與西伯利亞/雅庫茨克異常以及電漿匱乏灣日、季、緯度、高度之變化。福衛三號於2006年4月15日發射運行，由六顆近地衛星所組成，每顆衛星裝載「全球定位系統氣象量測儀」進行掩星觀測(GOX)，每天能提供全球平均2000筆電離層90至800公里高之電子濃度。
觀測結果顯示，威德海與西伯利亞/雅庫茨克異常發生的盛行高度約為距地300公里高。2007至2016年全球固定地方時研究結果顯示，南半球夏季子夜時段，高緯度之威德海異常仍具有最大之電子濃度，然而隨著地方時，此一異常峰會沿同一緯度，向東飄移之日變化週期現象；北半球西伯利亞/雅庫茨克異常所發生的中緯度，則同時出現兩個異常峰隨著地方時間有向東飄移之半日變化週期現象。水平風場模式Horizontal Wind Model 1993 (HWM93)模擬比對福衛三號觀測顯示，水平風引發沿著磁力線之磁子午風與威德海和西伯利亞/雅庫茨克異常位置吻合並同步飄移。結果顯示，磁子午風之子午赤道向分量與鉛垂向上分量是形成單一峰與雙峰的重要機制，同時證實這些異常現象全年全天候皆會出現。
福衛三號電子濃度觀測顯示，2007至2014年電漿匱乏灣盛行於赤道和低緯度電離層之275公里高。4月至9月間，北半球匱乏灣出現在30°–60°W (大西洋)、30°–110°E (印度洋)與120°–160°E (東南亞)三個緯度區域；10月至3月間南半球匱乏灣則會位於80°–150°W(南美洲西岸)，事實上四個匱乏灣於全年全天候皆會出現，且盛行高度會隨著地方時而改變。HWM93水平風場模擬，說明東西向之經向風造成沿磁力線之赤道向電漿飄移，是形成電漿匱乏灣的主要機制。整體而言，福衛三號觀察和HWM93水平風場模擬證實，中性風引發的磁子午向等效電漿飄移為造成威德海、西伯利亞/雅庫茨克異常、以及電漿匱乏灣的物理機制，而這些異常現象24小時全天候出現於一年四季。
;The Earth’s upper atmosphere, comprised of the thermosphere and ionosphere, is where neutral and charged particles interact causing complicated physical processes. The ionospheric electron density is highly variable with the altitude, latitude, longitude, local time, season, solar cycle. This dissertation shows the investigation of the nighttime features from the coupling between the ionosphere and thermospheric neutral wind. Two interesting phenomena associated with the electrodynamic processes are examined, which include (1) The Weddell Sea Anomaly (WSA) in southern mid to high-latitude and Siberia-Yakutsk Anomaly (SYA) in northern mid-latitude. The increasing anomalies of electron density are most prominent over the Weddell Sea region in the southern hemisphere and Siberia and Yakutsk areas in the northern hemisphere during local summer nighttime; and (2) The Plasma Depletion Bays (PDBs) at equatorial/low-latitude. These features of the electron density and TIMED/GUVI 135.6nm airglow emission are observed at the evening/night hours near magnetic equator in three longitude regions, North Atlantic, India Ocean, and Southeast Asia during May. Six microsatellites of the joint Taiwan-US satellite constellation mission, termed FORMOSAT-3/COSMIC (F3/C), were successfully launched in to a circle low Earth orbit at 01:40 UTC on 15 April 2006. Each satellite houses a GPS occultation experiment payload globally deriving the vertical electron density profile in the ionosphere. This constellation daily provides instantly more than 2000 profiles from 90 to 800 km altitude. Dense global electron density probing brings a new era of studying the space weather in the ionosphere.
In this dissertation work, the three-dimensional (3-D) plasma density structure constructed by electron density profiles from F3/C satellites are employed to study the diurnal, seasonal, latitudinal, and altitudinal variations of these anomalies and bay features. The results show that the WSA and SYA features occur prominently at about 300 km altitude, as well as yield the eastward shift of a single-peak plasma density along the WSA latitudes and a double-peak along the SYA latitudes during the period of 2007-2016. The thermospheric meridional and zonal winds simulated by Horizontal Wind Model 1993 (HWM93) is applied to interpret the plasma motions along the magnetic field lines associated with the WSA and SYA anomaly features. Results indicate that the meridional and vertical components of magnetic meridional wind can be responsible for the eastward shift of WSA single-peak and SYA double-peak plasma density. In fact, the WSA and SYA features constantly appear in whole day and all year round.
The PDB structures in the F3/C electron density prominently appear at 275 km altitude in the equatorial/low ionosphere. Three PDBs curving in the northern hemisphere around the magnetic equator situate in regions 30°–60°W (North Atlantic), 30°–110°E (India Ocean), and 120°–160°E (Southeast Asia) from April-September, while one PDB curving in the southern hemisphere appears in 80°–150°W (Southwest America) from October-March. A detailed study on the F3/C 3-D electron density structure shows that the four PDBs are intense mainly below the ionospheric peak density layer (~350 km altitude) in whole day and all seasons. A simulation of HWM93 suggests that the trans-equatorial plasma transports induced by the zonal wind result in the PDB features in the nighttime equatorial/low-latitude ionosphere. Blowing of the thermospheric neutral winds play an important role in the formation of the two anomalies and bay features. |
