| 摘要: | 地球高層大氣中之電離層和熱氣層，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、太陽活動及地磁活動，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、熱氣層與中氣層和磁層耦合及反饋作用。在低緯度電離層，由於磁力線近乎水平的結構，配合中性風及電磁場，而產生許多獨特的現象。本論文探討低緯度電離層在寧靜時期白天及日落後與電動相關之重要現象，包括有：(1)日間赤道電噴流，(2)白天電離層電漿垂直漂移，(3)日落前反轉增強(pre-reversal enhancement)垂直漂移。二個數值模式，美國國家大氣研究中心 (NCAR) 之熱氣層-電離層-電動環流模式(Thermosphere-Ionosphere-Electrodynamics General Circulation Model, TIE-GCM)和全球電離層電漿模式(Global Ionosphere and Plasmasphere Model, GIP)被用來探究上述現象之物理過程。在第一章介紹基本的電離層和熱氣層理論及其交互作用，以及低緯度之相關現象。第二章描述數值模式NCAR TIE-GCM和GIP的理論結構及計算過程。 赤道電噴流 (equatorial electrojet, EEJ)發生於赤道地區距地球表面100-120公里處，白天強大的東向電流流經電離層E層時，會對地表感應產生強大的磁場擾亂。計算赤道及北緯6-9度地區地表磁場水平分量的相差值(△H)，可推估赤道電噴流的強度。本論文之第三章利用TIE-GCM模擬秘魯(76°W)及菲律賓(121°E)地區△H之日變化、季節變化及太陽活動變化。模擬可重現△H和白天電漿垂直漂移之線性關係及雷達觀測之結果。這項研究的結果顯示，除了赤道電噴流會造成地表磁場擾動，中性風經由發電效應所產生的電流也會同時產生地磁的變動。因此，第四章進一步利用TIE-GCM研究在不同高度中性風影響下所造成的電離層電流變化，模擬春分時期及中度太陽活動下在秘魯地區地球磁場變化和電漿垂直漂移速度之線性關係，.模擬結果顯示低緯度地區的東西向中性風在不同高度的速度變化，足以改變南北緯6-9度地區的電流，而對地磁產生擾動，因此在利用地表磁場水平分量的相差值來推估電漿垂直漂移速度時，必須同時考慮赤道電噴流以及中性風發電效應對電離層造成的影響。 日落前反轉增強垂直漂移現象對夜晚電離層電子濃度及電離層探測儀觀測散狀F層 (equatorial spread F) 的產生具有相當大的影響，但至目前為止，此現象之詳細物理機制尚未有定論。由於TIE-GCM 能夠成功模擬此現象的發生產生，因此第五章利用TIE-GCM在中度太陽活動時期，監控在秘魯地區發生垂直漂移增強時電位、電場、導電率、電漿垂直漂移速度及中性風的變化，藉由上述各項電離層參數在不同經度上的變化以了解此現象產生時的空間結構，結果顯示電離層F層的中性風及日落後E層迅速減少導電率是產生此現象的主要原因，此外由中氣層向上傳遞的大氣潮汐也會對此現象有所影響。另外在此章節中更利用模式計算電流連續性，證實EEJ對日落前反轉增強垂直漂移現象之產生並無太大影響，利用數值模擬可以讓我們更有能力診斷此複雜電動力過程及驗證目前所提出之相關理論，以進一步了解夜晚低緯度電離層的變化。 第六章研究在固定當地時間下觀測電子濃度及電漿垂直向上漂移的四個極值，此模擬利用中華衛星一號(ROCSAT-1)觀測之電漿向上漂移速度配合GIP模式來模擬此四個極值的日變化及年變化，GIP模式成功地重現衛星在600公里之電子濃度觀測結果及其經度變化，四個極值的日變化和年變化模擬也與觀測之電漿垂直漂移及電子濃度呈現相同結果。最後在第七章中總結本論文。 The Earth’s upper atmosphere, comprised of the ionosphere and thermosphere, is where neutral and charged particles interact to cause complicated physical processes. It is partly because of the complexity that the region is still actively studied by scientists. The densities of charged particles show a significant variation with altitude, latitude, longitude, universal time, season, solar cycle, and magnetic activity. The variation results from changes in solar radiation and from the coupling and feedback mechanisms between the ionosphere-thermosphere system and the mesosphere and magnetosphere. In the low latitude ionosphere, the nearly horizontal geomagnetic field lines combine with the neutral wind and electric field to create some unique phenomena. This study focuses on the daytime and post-sunset low latitude ionosphere under geomagnetic quiet conditions. Several important phenomena associated with the electrodynamic processes are investigated which include (1) the daytime equatorial electrojet (EEJ); (2) the daytime ionospheric vertical drifts; and (3) the post-sunset pre-reversal enhancement (PRE) of the vertical drifts. Two different numerical models, the NCAR Thermosphere-Ionosphere-Electrodynamics General Circulation Model (TIE-GCM) and the Global Ionosphere and Plasmasphere Model (GIP), are utilized to examine and understand the physical processes of these phenomena. In Chapter 1, fundamental theories of ionosphere and thermosphere, their interaction and associated phenomena in the low latitude region are introduced. Structures and electrodynamic calculations of the TIE-GCM and the GIP model are described in Chapter 2. Between 100 and 120 km height at the Earth's magnetic equator, the equatorial electrojet flows as an enhanced eastward current in the daytime E region ionosphere which induces a magnetic perturbation on the ground that is considerably larger than the perturbations at neighboring latitudes. Calculating the difference between the horizontal components of the magnetic perturbation (H) at magnetometers near the equator and about 6-9 degrees away from the equator, △H, provides us with information about the strength of the EEJ. The TIE-GCM is capable of simulating the EEJ current and its magnetic perturbation on the ground. In Chapter 3, the simulated diurnal, seasonal, and solar activity variations of △H in the Peruvian (76°W) and Philippine (121°E) sectors, and the relation of △H to the ionospheric vertical drift velocity, are presented. Results show the diurnal, seasonal and solar activity variations are captured well by the model. Agreement between simulated and observed magnitudes of △H and its linear relationship to the vertical drift is improved by modifying the original daytime E region photoionization in the TIE-GCM in order to better simulate observed E region electron densities. The model results suggest that besides the electric-field-driven EEJ, wind-driven currents can also be an important factor in changing the magnitude of △H. Therefore, we use the TIE-GCM to further examine the influence of the neutral wind. In Chapter 4, we run the model at the March equinox under moderate solar activity (F10.7 = 140 units) and modify the neutral wind and high latitude electrical potential to obtain the latitudinal distributions of H, diurnal variations of △H and daytime vertical drift in the Peruvian (75°W) longitude sector. The relationship between △H and vertical drift is also simulated and analyzed, which helps us to understand the importance of both the EEJ and the off-equatorial wind-driven currents in altering the relation. Simulations show that a height-varying wind velocity in the low latitude region is capable of modifying the ground magnetic perturbation a few degrees away from the equator, while affecting the equatorial perturbation only little. Only by combining the effects of both the EEJ and the off-equatorial wind-driven currents can the magnitude of △H and its relation with the vertical drift be accurately estimated. The pre-reversal enhancement is one of the most important phenomena controlling the nighttime ionosphere and the generation of equatorial spread F (ESF), but its causal mechanism is still not fully understood. The TIE-GCM is capable of simulating ionospheric phenomena and the pre-reversal enhancement. In Chapter 5, the model is run under moderate solar activity (F10.7=150) and geomagnetic quiet conditions to monitor the variation of ionospheric parameters when the maximum upward drift of the PRE is occurring in the Peruvian longitude (75°W). Since it is a three-dimensional model, it provides us electric potential, electric field changes, conductivity variations, ion drifts, and neutral wind at the Peruvian longitude and also at longitudes to the east (post-PRE) and west (pre-PRE). From the results, the spatial variations of the ionosphere surrounding the PRE can be illustrated and effects due to various physical parameters are evaluated. Specifically, the effects of neutral winds and conductivities separately in the ionospheric E and F regions on the PRE are examined. Results show that the F region wind and sudden decrease of E region conductivities are essential for creating the PRE. The upward propagating thermospheric tides also participate in the formation of the PRE. Through calculating the current divergence, result demonstrates that the contribution of the EEJ to the F-region upward current and the vertical drift is negligible. With the TIE-GCM simulation, we are able to diagnose the complex electrodynamic process to examine the existing theories and to further understand the morphology of nighttime low-latitude ionosphere. In Chapter 6, we investigate the annual and local time variations of the wave-4 structures in the plasma density and vertical drift in the low-latitude F region by analyzing the measurements from the first Republic of China SATellite (ROCSAT-1) and conducting simulations with the GIP model. In order to understand how the vertical drifts relate to the longitudinal structure of the topside ionosphere, we apply the equatorial vertical drifts observed from ROCSAT-1 to drive the GIP model. The model well reproduces the longitudinal structure in electron density, and the magnitudes of electron density are comparable with ROCSAT-1 measurement at 600 km. The annual and local time variations of the wave-4 component in the GIP model density also show good agreement with the vertical drift and plasma density observed by the ROCSAT-1. Finally, Chapter 7 summarizes the main conclusions of this work and identifies future research directions. |
