| 摘要: | 摘要 在赤道區電離層F 層不規則體(F-region irregularities) 被廣泛 地觀測與數值模擬研究後，對於其產生的機制已逐漸地瞭解，基本 上是由GRT 不穩定性(Gravitational Rayleigh-Taylor Instability)，再 加上B E v v × 梯度漂移不穩定性( B E v v × Gradient Drift Instability)，兩者 的效應造成電離層F 層的不規則體。此種不規則體是由電離層底部 產生的擾動，受到GRT 與B E v v × 不穩定性的放大效果逐漸往上舉升， 最後穿過電離層最大密度區(F-region peak)。但由衛星AE-C 與 AE-E 的觀測資料，發覺有一種F 層底部正弦曲線不規則體 (Bottomside Sinusoidal (BSS) Irregularity) 的存在，對於此種不規則 體因為缺乏數值模擬的研究，所以對其產生機制並不瞭解。因此本 篇主要目的之一是要瞭解此種不規則體的形成機制。 本篇另一個主要目的為瞭解電離層不規則體發生的經度與季節 的關係與磁暴對控制電離層不規則體的形成的兩個研究。因為全球 定位系統(Global Positioning System, GPS) 的地面觀測站分佈於全 球，適合研究全球電離層不規則體發生的經度與季節的關係。另外 在磁暴與電離層不規則體的研究，為了排除其他控制電離層不規則 體形成的影響，所以選擇不規則體不易產生的季節，當磁暴發生時， 利用全球定位系統在中南美洲的地面觀測站來分析電離層的反應。 對於F 層底部不規則體的研究主要是利用數值模擬的方法，由 二維電漿流體模擬程式(Fluid model simulation code) 來模擬此種 不規則體產生的條件。由International GPS Service (IGS) 所提供的 資料，來研究電離層不規則體發生的經度與季節的關係與磁暴對控 制電離層不規則體的形成。對於前者我們選擇1998 年的整年資料來 分析。對於後者選擇從1997 至2000 年五月至八月發生的磁暴(五 月至八月期間，對於中南美洲而言，是不規則體不易產生的季節) 來 分析電離層的狀況。利用全球定位系統雙頻虛擬距離與載波相位觀 測的組合來求得電離層全電子含量值(Total Electron Content, TEC)，再由全電子含量值在時間上的變化量，得到由電離層不規則 體所造成的全球定位系統相位擾動。由實際觀測天數與有相位擾動 發生的天數得到的統計資料，來分析電離層不規則體發生的經度與 季節的關係。另外根據磁暴發生時，地磁指數Dst 變化的情形與觀 測得到的相位擾動之間的關係，來瞭解磁暴控制電離層不規則體的 情形。 由二維電漿流體模擬程式模擬不同的電離層外在的環境，最後 終於成功地找出電離層F 層底部不規則體形成所需要的環境。當電 離層底部產生的擾動，受到GRT 與B E 不穩定性的放大效果逐漸 往上舉升，此時若在電離層最大密度區下存在一層噴射氣流(垂直風 切)，因為動力不穩定性的關係，會把擾動限制在電離層底部發展而 不會繼續往上，因此形成電離層底部不規則體。 在研究電離層不規則體發生的經度與季節之間的關係，我們得 到在大西洋區電離層不規則體的發生頻率，冬季(5 月至8 月)比夏季 (11 月至隔年2 月)甚少，在太平洋的區域，則有相反的結果。磁暴 與電離層GPS 相位擾動之間的研究，經過八個磁暴資料的分析得到 地磁指數Dst 變化的時間與全球定位系統相位擾動的產生有相當程 度的關係，只有當地磁指數Dst 的數值在當地的日落前後開始劇烈 地下降，才會造成強烈的相位擾動，另外磁暴的強度也是控制電離 層不規則體發生的因素之一。 After extensive research efforts in both observations and theoretical simulation, it is generally believed that the F-region irregularities in the ionosphere above the equator are generated by the combined effects from the gravitational Rayleigh-Taylor (GRT) instability and the ExB gradient drift instability. Such generated irregularities initiate near the bottom of ionosphere due to small disturbances. They are amplified by GRT and ExB and gradually move upward, eventually penetrate the F-region peak of the ionosphere. However, observations from Satellites AE-C and AE-E suggest the existence of F-region bottomside sinusoidal (BSS) irregularities. Due to the lack of numerical modeling on such features, the mechanism responsible for their occurrence is not well understood, which is one of the purposes of this study. Another goal of this study is to understand the relationship between the occurrence of ionospheric irregularities and controlling factors such as the longitude, the season, as well as the existence of magnetic storms. The worldwide distribution of Global Positioning System (GPS) stations enables such studies. I analyze the ionospheric characteristics in mid- and south- America using GPS signals to study the effect of magnetic storm on the generation of irregularities, specifically choosing the low-occurrence season to prevent the effects from other factors. The study on the occurrence of F-region BSS irregularities is done through numerical simulation using the two-dimensional fluid model simulation code. The GPS data, which is used to study the relationship between the occurrence of irregularities and factors such as the longitude, season, and the existence of magnetic storms, are provided by the International GPS Service (IGS). For the formal, we select the entire year of 1998 for analysis, while for the later we choose magnetic storms that occurred in May-August (i.e., the low-occurrence season in mid- and south-America) between 1997 and 2000. Utilizing the information from dual-frequency pseudo range measurements and the carrier phase observations, the total electron content (TEC) can be estimated. Consequently, the variation of TEC with respect to time gives the GPS phase fluctuation due to the ionospheric irregularities. The relation between the occurrence of irregularities and the longitude and season can be then derived from the statistics on the number of days when irregularities are observed versus the total number of observation days. Furthermore, the effect of magnetic storms on the generation of irregularities can be delineated from the time sequence of Dst index and phase fluctuations. Our simulation results indicate that specific environment is necessary for the occurrence of F-region BSS irregularities. When the seeding disturbance near the bottom side of F-region moves upward due to the amplifying effects from GRT and ExB instabilities, the dynamic instability would confine the generated irregularity near the bottom of ionosphere if there is a jet stream (i.e., vertical wind shear) immediately below the F-region peak of the ionosphere, resulting the F-region BSS irregularities. As for the relationship between the irregularities and longitude/season, our results indicate stations in the Atlantic have high occurrence rate in winter (May-August) than in summer (November-February). In contrast, stations in the Pacific have the opposite pattern. Our study on the 8 magnetic storms indicates significant correlation between the time variation of Dst index and the GPS phase fluctuations. Strong phase fluctuations can be observed only when Dst index drops rapidly during the time of sunset. Furthermore, the intensity of the magnetic storm is another factor that controls the occurrence of ionospheric irregularities. |
