福衛三號(FORMOSAT-3/COSMIC)衛星群上的GPS接收機提供每日約2000個掩星觀測。有別於單顆衛星或地面電離層觀測，由這些掩星觀測量能反演出真正涵蓋全球及高度分布的電離層資料。因此本論文將利用其在2007及2008年間所提供的電離層電子濃度與閃爍指標S4資料做三維空間分布的統計分析，以瞭解電漿不規則體之全球與高度的分布。我們將特別鎖定大西洋經度 (0°~75°W)涵蓋地磁最弱區(SAA)之範圍(簡稱SAA經度範圍)以研究磁赤道區F層電離層不規則體的型態與機制。我們之所以有興趣於此範圍是因為其獨特的地磁結構及其經常有高能粒子沉降在SAA區發生。 統計分析福三資料結果顯示: (1) 在太陽活動極小期與地磁活動寧靜期的狀態下，日落後EFI/閃爍事件只集中在北半球冬季(D-months)之大西洋經度範圍，而北半球夏季(J-months)在SAA經度範圍卻幾近於無。(2) 北半球冬季赤道區F層之電漿不規則體平均而言分布於赤道區F層底部(200~350公里高)和接近於全球最大Nmax和Hmax 所在的緯度位置。(3) 日落線、磁偏角、強閃爍事件和電子濃度消散區之間有存在極強的空間相關性只有當在北半球冬季之大西洋經度範圍，因該區是磁場方向與E層日落線最接近平行的地方。(4)由太陽輻射和下降高能粒子所激發的導電率梯度會造成赤道區F層電場之增強。(5) 當在北半球冬季時，電子濃度的橫向梯度(西向和南向)及水平的中性風(東向和北向)的交互作用下對EFI的成長率有加成效果。故由COSMIC觀測資料可知對於寧靜期赤道區F層電漿不規則體的發生率與分布隨季節及經度的變化，除了受日落赤道區電動力的控制外，SAA的下降高能粒子也扮演著極關鍵性的角色。所以本論文充分證明福三連續性的全球掩星觀測能提供電離層不規則體研究非常有效的三維資料。在太空天氣的研究上，了解赤道區的電離層不規則體的形態與發生機制是個非常重要的課題。The observations of the GPS occultation sensors onboard the six constellation satellites of COSMIC/FORMOSAT-3 are able to provide the truly global measurements of the ionosphere every day (~2000 occultation measurements). The large data set enables us a statistically correlative study of electron density depletion and GPS L1 scintillation, thus allows the global and altitudinal distributions of irregularity to be examined. In this thesis we will focus on the longitude sector (0o ~75oW) where the South Atlantic magnetic Anomaly (SAA) is located to study the morphology of equatorial F-region irregularity (EFI). The SAA longitude sector is selected because of its unique geomagnetic configuration and because of the constant existence of energetic particle precipitation in SAA. From the statistical analyses of the ionospheric electron density and scintillation S4 index data from FORMOSAT-3/COSMIC during 2007~2008, we have obtained the following results: (1) Under solar minimum and geomagnetic quiet conditions, post sunset EFI/scintillation events are found to concentrate in the SAA longitude sector during northern winter months (D-months), but nearly disappeared in the same longitude sector during the opposite season (J-months). (2) The D-months’ average pattern of EFI reveals that most of irregularities occurred in the bottom side F-region (200~350 km altitudes) and at low magnetic latitudes adjacent to the locations of the maximum post-sunset Nmax and Hmax. (3) Strong spatial correlations are found to exist among the sunset terminator, the westward declined geo-magnetic field, strong scintillation event, and density depletion in the SAA longitude sector only during D-months, where the E-region sunset terminator is most possibly parallel to the magnetic field direction at equatorial latitudes. (4) Two sources of ionization are considered to likely contribute to the post-sunset electric field enhancements at equatorial latitudes of the SAA longitude sector by means of creating ionospheric conductivity gradients. The first is solar radiation which can cause longitudinal conductivity gradients near the sunset terminators. The second is energetic particle precipitation which can cause conductivity gradients in the vicinity of SAA. (5) The combination of horizontal density gradients with anti-parallel (north-eastward) neutral winds during D-months can contribute additional growth rate to the EFI generation. These COSMIC observations not only confirm that the sunset equatorial electrodynamics plays a key role in controlling the seasonal and longitudinal occurrences of the quiet time EFI, but also reveal that seasonally dependent ionospheric responses to the energetic particle precipitation in SAA can affect considerably the morphology of EFI in the SAA longitude sector. We have demonstrated in this thesis that the continuously global occultation measurements from COSMIC/FORMOSAT-3 provide useful 3D data for the study of ionospheric irregularities. The morphology and the generation mechanisms of the equatorial F-region irregularities are critically important to the space weather research.
