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    Please use this identifier to cite or link to this item: http://ir.lib.ncu.edu.tw/handle/987654321/5068

    Title: 以HHT時頻分析法剖析赤道區電離層F層電漿空腔結構;Anatomy of ESF plasma structures by using HHT time-frequency analysis
    Authors: 陳崗熒;Kang-Ying Chen
    Contributors: 太空科學研究所
    Keywords: HHT時頻分析法;電離層F層電漿空腔結構;低頻漂移波;雙極電場;HHT time-frequency analysis;Equatorial Spread F;low frequency drift wave;ambipolae electric field
    Date: 2001-07-13
    Issue Date: 2009-09-22 09:44:29 (UTC+8)
    Publisher: 國立中央大學圖書館
    Abstract: 本文主要針對赤道地區電離層F層電漿空腔(ESF)之小尺度擾動結構進行剖析與研究,利用華衛一號IPEI酬載沿著飛行軌道所提供的高解析度之電漿濃度與速度資料,配合新的時頻分析法”HHT”,可以在不需做線性化假設的情況下,直接將電離層離子速度與濃度梯度之一維空間資料利用EMD過程將之分解成若干個具有不同特徵尺度的基模(IMF),之後再對各個基模施以HT進行時頻(空間波數)分析。 配合一套吾人由HHT衍生發展出來的尺度相關分析法,發現在電漿泡內部的結構中,擾動尺度約300m 以下的離子垂直速度擾動與濃度橫向梯度擾動彼此之間具有極相似的擾動波形,這意謂著波長尺度在300m 以下的電漿結構擾動中,能引發離子垂直運動的橫向擾動電場必須滿足波茲曼關係式。 另外,透過HHT波譜分析吾人發現,離子垂直速度波譜在50m附近出現明顯快速的轉折,並且與相對濃度梯度波譜有明顯差異。 配合理論進一步研究發現,300m至50m的區間,主要的機制為低頻漂移波;而50米以下的擾動則很可能是離子擴散機制產生雙極電場的效應所造成的。 本論文使用IPEI所提供的高解析資料,並引進新的分析技術HHT,成功有效的對赤道地區電離層F層之小尺度電漿結構進行空間域與波數域的剖析,一方面確認出低頻漂移波在300m至50m附近的尺度區間扮演主要機制,並指出50米以下的擾動則很可能是離子擴散機制產生雙極電場的效應所造成的;另一方面則發現離子垂直速度的HHT波譜在50m附近有”二次轉折”的現象存在,透過若干案例的分析與歸納發現此一新現象具有“相對濃度梯度越強,速度波譜發生轉折時所對應的波長越小; 而波譜轉折發生時所對應的能量水位亦較高”的特性。 This paper investigates the small scale plasma structures observed by ROCSAT-1 in the equatorial F region through the newly developed Hilbert-Huang transform (HHT) method in the time (space) domain under the frozen-in approximation. The new method allows us to decompose the non-stationary, nonlinear data into a finite number of intrinsic scale modes. In this thesis the structures of vertical ion velocity and horizontal density gradient inside a plasma bubble are analyzed mode by mode anatomically without making the usual linearization assumption. We found that the intrinsic modes for velocity and density gradient of the selected event have identical wave form for structures with scales between 300 m and 50 m. This implies that the vertical velocity fluctuations induced from the electric field follows the exact Boltzmann relation in the limited regime of scale length between 300 m and 50 m. Further studies show that the low frequency drift wave should be dominant in these scale lengths. Meanwhile, a spectral break at 50 m is clearly seen in the velocity HHT spectrum. The spectral shape of velocity fluctuations differs greatly from that of density gradient at scale lengths below 50 m. Hence the ambipolar electric fields induced by the ion diffusion are proposed, and such a process could be important in driving ESF small-scale fluctuations at scale lengths below about 50m.
    Appears in Collections:[太空科學研究所 ] 博碩士論文

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