低緯度電離層不規則體之GPS相位擾亂長期觀測

A long term observation of ionospheric irregularities at low latitudes by using GPS phase fluctuations

褚芳達(Fang-Dar Chu)

指導教授 ： 劉正彥

國立中央大學/地球科學學院/太空科學研究所/博士(2007年)

摘要

電離層的電子密度不規則體會影響電磁波的傳播，赤道地區夜間電子密度不規則體所引起的散狀F層現象之發生機率的全球分布一直是重要的研究主題。由過去多種觀測技術 (radar power map / scintillation, ionosonde spread-F, satellite measurement / scintillation, and optical imager) 可推論出赤道散狀F層 (ESF) 的發生率有明顯的經度效應；但那些觀測系統彼此之間的差異性大，各有其限制且判斷有不規則體存在的criteria也各不相同，觀測結果只能做定性比較；而且那些觀測資料通常也不夠長或不夠密集而無法顯現整個太陽週期之ESF發生率的變化。
本研究有兩個目的，一個目的是發展一種新觀測技術，只使用全球定位系統 (GPS) 就能在全球各地進行全天候長期觀測，且以相同的分析方法與criteria來判斷不規則體是否發生；另一個目的是探索全球低緯度各經度區的ESF發生率，以及提供整個太陽活動週期的ESF發生率逐年演進細節。研究方法首先是研發以傾斜全電子含量計算GPS相位擾亂的analysis approach，由於不需要儀器偏差值，所以可以直接應用到所有GPS觀測資料；然後以GPS相位擾亂偵測全球低緯度地區的ESF，統計其發生機率並比較各經度區的差異。
研究結果顯示，(1) 赤道散狀F層的occurrence pattern有非常顯著的經度效應；太平洋的Kwajalein、Guam、菲律賓，以及非洲地區是equinox and June solstice months 發生率高，但南美洲的巴西、祕魯、以及印度卻是equinox and December solstice months 發生率高。(2) 在太陽活動相關性方面，強烈不規則體發生率與太陽活動呈正相關；而普通不規則體發生率通常也與太陽活動呈正相關，只有兩個例外是巴西的December solstice months與太陽活動無關，以及祕魯的equinox months 與太陽活動無關或僅呈弱相關。(3) 至於ESF發生率之逐年演進則發現不規則體發生率在那個季節月份最大是取決於當地 equinox months 與 June or December solstice months 發生率的拉鋸競爭，在不同太陽活動階段常會有不同結果，只有兩個例外是巴西 (始終都是December solstice maximum) 與菲律賓 (始終都是equinoctial maxima)。
本研究發展完成的GPS相位擾亂analysis approach可實現只用一種觀測系統與分析方法就能進行全球全天候長期觀測的目的，讓GPS成為觀測電離層不規則體的有用工具。而且本研究為首次嘗試分析全球低緯度地區長期大量GPS觀測資料且已獲得豐富結果，能描繪較完整可靠的全球ESF發生率分佈情況。

關鍵字

電離層不規則體 ； 赤道區電離層

並列摘要

The electron density irregularities in the ionosphere would affect the propagation of electromagnetic wave. The spreading F layer phenomena which caused by electron density irregularities at low latitudes is an important research topic. It could have been deduced that the distribution of occurrence rates of equatorial spread F (ESF) shows obvious longitude effect by using many observation techniques (radar power map / scintillation, ionosonde spread-F, satellite measurement / scintillation, and optical imager) in the past. Nevertheless, those techniques are not only very different each other, but also with their individual limits and with different criteria to detect the existence of ionospheric irregularities. The results obtained by those techniques would only be made qualitative comparisons. Besides, those observation data are usually not sufficient long or not continuous to show the year by year variation of ESF occurrence rate during an entire solar cycle period.
There are two objectives in this paper. One objective is to develop a new observation technique that can conduct a global, all weather, and long-term observation of the ionosphere by using the global position system (GPS), and moreover, can detect the existence of ionospheric irregularities with the same analysis procedure and criterion. The other objective is to investigate the distribution of ESF occurrence rates at low latitudes, and moreover, investigate the details of year by year evolution of ESF occurrence rates during an entire solar cycle period. The method was as the following. First, a new analysis approach of GPS phase fluctuation was developed by using slant total electron contents, rather than vertical total electron contents. The new analysis approach needed no instrumental biases, and therefore it could be directly applied to every GPS observation data set. The last, the new GPS phase fluctuation approach was applied to detect the ESF occurrence at global low latitudes, and moreover, the statistics of ESF occurrence were calculated and the comparisons of the occurrence rates between different longitudes were made.
There were three important results. (1) The occurrence distribution of equatorial spread F did show obvious longitude effect. The occurrence rates peak at equinox and June solstice months in the Pacific sector (Kwajalein, Guam and Philippines) and Africa, nevertheless, the occurrence rates peak at equinox and December solstice months in South America sector (Brazil and Peru) and India. (2) Strong ionospheric irregularities are always positive dependent on solar activity; and similarly, moderate irregularities are usually positive dependent on solar activity, excepting that those in Brazil are independent in December solstice months, and those in Peru are independent or only weak dependent in equinox months. (3) As to the evolution of occurrence rates of irregularities with solar activity, it should be noted that irregularity occurrence rates peak in which season (month) are determined by the competition between the occurrence rates in equinox months and June or December solstice months, and therefore, would be different results in different phase of solar activity, excepting for Brazil (always December solstice maximum) and Philippines (always equinoctial maxima).
The GPS phase fluctuation analysis approach developed in this study does satisfy the objective to investigate the global ionosphere with the GPS system with all weather and long term observations. This study is the first attempt to analyze long term GPS observation data for the research of equatorial spread F layer. Many interesting results have been obtained, and a more complete and reliable longitude effect on equatorial spread F layer could be portrayed.

並列關鍵字

ionospheric irregularities ； equatorial ionosphere

參考文獻

Aarons, J. (1993), The longitudinal morphology of equatorial F-layer irregularities relevant to their occurrence, Space Sci. Rev., 63, 209-243.

Abdu, M. A., J. H. A. Sobral, O. R. Nelson, and I. S. Batista [1985], Solar cycle related range type spread-F occurrence characteristics over equatorial and low latitude station in Brazil, J. Atmos. Terr. Phys., 47, 901-905.

Anderson, D. N., B. Reinisch, C. E. Valladares, J. Chau, O. Veliz (2004), Forecasting the occurrence of ionospheric scintillation activity in the equatorial ionosphere on day-to-day basis, J. Atmos. Solar Terr. Phys., 66, 1567-1572.

Basu, S., J. Aarons, J. P. McClure, C. La Hoz, A. Bushby, and R. F. Woodman (1977), Preliminary comparisons of VHF radar maps of F-region irregularities with scintillations in the equatorial region., J. Atmos. Terr. Phys., 39, 1251-1261.

Booker, H. G., and H. W. Wells (1938), Scattering of radio waves by the F region ionosphere, J. Geophy. Res., 43, 249-256.