- 學位論文
哨聲波在偶極場及長尾場模式的傳播特徵
The Propagation Characteristics of Whistler-Mode Waves in Dipole and Tail-Like Field Models
摘要
在磁副暴期間地球磁尾的偶極化過程中，哨聲波已經被觀測到並由一些文獻報導。然而，這些波是否可以藉由傳播的方式來影響地球的內磁層區域尚未被詳細探討。因此，本論文研究以射線追蹤技術，計算了由磁尾赤道處10 個地球半徑出發的哨聲波的傳播特性。我們設計了三種不同形狀的磁場模型及電漿密度模型來模擬偶極化過程中的不同階段，來計算頻率範圍從 10 Hz 到 800 Hz 且初始波動角度為 0 至 60度 的哨聲波的傳播路徑。計算的結果可以總結出四個最主要的特徵: 也就是在初始波動向量指向地球外的情況下，首先，低頻波比高頻波更有可能穿透電漿層頂。其次，長尾場結構可以使波動更容易到達電漿層頂。第三，利用由擴散平衡模型和電漿片模型組成的密度模型，可以使波動比其他兩種密度模型更有可能進入電漿層頂。最後，具有較大初始波動角的哨聲波較易向內層區域傳播。這些傳播特性基本上可以低混合頻率和折射率的空間分佈來解釋。上述結果顯示磁副暴活動期間於磁尾區產生的低頻率的哨聲波有可能影響到計靠近地球的電漿層區域。
並列摘要
Several reports have shown that whistler-mode waves occurred during dipolarization at Earth’s tailside at substorm time. However, whether these waves can propagate to impact the inner magnetosphere has not been investigated thoroughly. Therefore, by implementing the raytracing technique, this study investigates the propagation characteristics of whistler-mode waves from 10 Earth radii at the equator. Three different magnetic field configurations and plasma density models are modeled to simulate different stages during dipolarization. Ray paths of waves with frequencies ranging from 10 Hz up to 800 Hz and initial wave angles from 0o to 60o are computed. The results show four distinct characteristics under the condition when the wave vector initially points outward from the Earth. First, low-frequency waves are more likely to penetrate the plasmapause than high-frequency are. Second, the tail-like field structure can allow waves to reach the plasmapause more easily. Third, using the density model composed of the diffusive equilibrium model and the plasma sheet model can enable waves with more possibility to access the plasmapause than those in the other two density models. Last, waves with larger initial wave angles can propagate more inward toward Earth. These propagating characteristics are explained fundamentally by the spatial distributions of the lower hybrid frequency and the refractive index. The above results suggest that low-frequency whistler-mode waves generated at the tail region during substorm activities may affect the plasmasphere.