臺灣博碩士論文加值系統

本研究主要利用福爾摩沙衛星三號在電離層中的的電子密度觀測資料來討論電波掩星觀測法在低緯度地區與地面觀測站的垂直觀測法之間的誤差關係。使用的地面觀測站資料來自中太平洋馬紹爾群島的爪加林觀測站。
電波掩星觀測法是用來觀測電離層垂直分布的一種方式，主要先使用低軌道衛星接收全球定位系統衛星測得時間延遲來測量由全球定位系統衛星到低軌道衛星之間訊號路徑上的電子總含量，再用阿貝爾反演法以電子總含量來反演垂直分布的電子密度。阿貝爾反演法有一個重要的假設，就是假設反演區域的電子密度為球對稱，但是實際上電離層不是完美的球對稱，而且在掩星過程電離層的電子密度也不是固定不變。比較方法為利用福衛三號的垂直電子密度分佈圖計算垂直方向上的電子總含量，與地面觀測站垂直電子密度分佈圖計算出的垂直方向電子總含量相減，以地面觀測站當作基準去計算誤差百分比。使用不同的參數與此誤差百分比做比較，運用的參數有當地時間，不同的高度區間，最大電子密度的高度，最大電子密度所在經緯度與地面觀測站所在經緯度之間的夾角(δ角)，最大電子密度經緯度與地面觀測站經緯度的連線與地面觀測站觀測方向間的夾角(α角)，Dst指數，Kp 指數，F10.7 指數，還有掩星觀測路徑所經過赤道異常區Wave-4中的中太平洋區域的比例。
我們發現兩個不同測量電子密度方法的誤差主要會因為α角度越大而誤差越大，而次要原因則是因為掩星觀測路徑經過Wave-4的中太平洋區域的掩星路徑比例，在α角越小時，經過Wave-4的中太平洋區域比例越高，誤差也有相對越高，但α角越大，主要影響誤差的原因又回到α角。這些原因主要都是因為阿貝爾反演中法球對稱的假設而造成的誤差。

In this study, we are going to use the electron density data in the ionosphere of FORMOSAT-3/COSMIC and the ground observatory electron density data to compare the difference between the two electron density measurements. The ground observatory is using the Kwajalein observatory at the Marshall Islands in Central Pacific Ocean.

Radio Occultation is a method to measure the vertical distribution of the atmosphere and the ionosphere, using the time delay of the signals received by Low-Earth-Orbit (LEO) satellites from the GPS satellites, to calculate the calibrated total electron content, then using Abel inversion to retrieve the vertical electron density. The Abel inversion algorithm is based on an important assumption, of spherical symmetry in the density. But actually, the density in the ionosphere is not spherical symmetric and may not be steady during the occultation event. Violation of this assumption may cause some inaccuracy in the density observation. The way we compare the discrepancy of two electron density measurements is to calculate the vertical total electron content measured by FORMOSAT-3/COSMIC and ground observatory, then to use the result from the ground observatory as the standard to calculate the percentage error between the two results. The percentage errors are then examined along with several parameters, such as local time, different altitude ranges, the maximum electron density altitude, angles (δ and α), Dst index, Kp index, F10.7 index, and the ratio of the occultation path length inside one of the Wave-4 region, Central Pacific Ocean, to check whether any of the parameters can affect the percentage difference between the two measurements methods.
We find that the percentage error mainly depends on the angle , larger angles correponding to larger percentage errors, and the secondary reason is the ratio of the occultation path length inside the Central Pacific Ocean of Wave-4 region. These reasons are related to the spherical symmetry assumption which Abel inversion relies on not being satisfied.

摘要.............I
ABSTRACT.............II
ACKNOWLEDGMENTS.........IV
CONTENT.............V
CHAPTER 1 INTRODUCTION.........1
1.1 Research Motivation..........1
1.2 FORMOSAT-3/COSMIC........2
1.3 GOX of FORMOSAT-3/COSMIC........4
1.4 Gorund Data Sources.........5
1.4.1 Kwajalein Observatory (KJO)......5
1.4.2 World Data Center for Geomagnetism, Kyoto.....6
1.4.3 OMNIWeb Data Explorer-NASA......8
CHAPTER 2 DATA PROCESSING.......10
2.1 Data Selection ...........10
2.2 Radio Occultation Method........10
2.3 Total Electron Content (TEC) calculation.....12
2.4 ALTAIR electron density measurement......20
2.5 Angle...........24
2.6 Correlation coefficient r.........29
2.7 Linear regression.........32
CHAPTER 3 DATA ANALYSIS AND DISCUSSION OF RESULTS..36
3.1 Local Time Comparison........36
3.2 Altitude Comparison.........37
3.3 Angle Comparison.........40
3.4 Geomagnetic Parameters Comparison......44
3.4.1 Dst and ∆Dst index comparison......44
3.4.2 Kp index comparison.........46
3.4.3 F10.7 index comparison.......46
3.5 Wave-4 patterns region comparison......47
3.6 Discussion of Results..........52
CHAPTER 4 CONCLUSION.........55
Reference.............56
Appendix.............58

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