一般的電漿研究通常都只針對電子和質子存在的理想環境去作分析，但是實際在大自然中，尤其是地球的電離層，甚至是實驗室裡的分析儀器，多離子電漿是存在的，當多離子電漿的離子百分比不同，會造成不同的物理現象，因此研究多離子電漿是很重要的。交會頻率只會發生在波動向量平行於背景磁場時。當兩種波模相交在交會頻率時會改變極化特性，例如從右手哨聲波轉換到左手離子迴旋波。交會頻率出現在多離子電漿中時，會因為背景離子的組合種類和所佔之濃度百分比變化而變化。由於波的極化特性會影響其傳播性質，分析這些頻率的發生值是很重要的。在本論文中，將研究至含有三種離子的電漿中的交會頻率 (crossover frequency)。本研究的最後，會利用DEMETER衛星上的質子哨聲 (proton whistlers) 資料計算各種離子在該觀測點內所占的濃度百分比，還有在該百分比下所對應的交會頻率，最後再將交會頻率對質子迴旋頻率歸一化 (normalize)，以利日後分析與應用。

The crossover frequency in plasma is a frequency at which the wave can change polarization and convert to another mode such as from right-hand whistler-mode to left-hand ion-cyclotron waves when the wave vector is parallel to the field lines. This frequency occurs in multi-ion plasma and their values would vary as the compositions/percentages of background ion species vary. In the thesis, we not only derive the 2-ion and 3-ion plasma’s formula of the crossover frequency but also use the crossover frequencies of ion whistlers observed by DEMETER satellite to calculate the percentages of each ion species at the observational site. Finally, for further application to analyze data from satellite missions, the crossover frequencies over different percentages for each ion species are normalize to proton gyrofrequency.

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