依據前人研究，已知火山噴發後造成大氣之擾動可以形成聲波在高空大氣傳遞。在亞電離層(subionosphere)的高度下，藉由雙頻全球衛星定位系統的量測可以了解中性粒子和自由電子之間耦合所造成電子濃度之變化。在西元2012年11月21日，位於紐西蘭之Mount Tongariro火山在國際標準時間零點二十分開始噴發，藉由紐西蘭GPS觀測站紀錄資料之分析，發現在噴發之後開始約10分鐘，觀測到有擾動波自火山噴發位置遠離。結果顯示，在11月21日出現以同心圓方式從火山中心向外傳遞的擾動現象，觀測到的水平波速從145m/s至684m/s不等。經研究分析為火山噴發所形成之大氣波主要有聲波及重力波的形式，而前人研究觀測之擾動速度屬於聲波，符合MSIS-E Atmosphere Model [Hedin, 1987,1991]模擬聲波在亞電離層高度下之波速；而本研究觀測之擾動為重力波，並經由小波分析(Wavelet Analysis)發現聲波頻率轉變成重力波之過程，符合理論聲波和重力波在等溫大氣下在亞電離層高度下臨界頻率之分布，呈現火山噴發形成重力波之現象。最後藉Atmosphere Ionosphere Perturbation Model進一步確認觀測結果，推論電離層火山噴發所造成的擾動主要為是能量變化所產生的而非火山噴發造成地表垂直速度變化所造成。本研究成果有助於進一步了解聲重力波之形成與火山噴發對電子濃度擾動之機制。

On November 21 2012, the explosion of the Mount Tongariro volcano in New Zealand occurred at UT 0:20. The New Zealand dense array of Global Positioning System recorded ionospheric disturbances as changes in Total Electron Content ~10 minutes after the eruption, and the concentric spread of disturbances also can be observed this day. Through using the spectral analysis of the rTEC time series in this study shows two peaks. The larger amplitudes are centered at 700 and 1400 seconds, in the frequency range of acoustic waves and gravity waves. The result supports the distribution of the periods of the acoustic waves and gravity waves at subionospheric height in theory. On the other hand, to model the rTEC perturbation created by the acoustic wave caused by the explosive eruption of the Mount Tongariro, we perform acoustic ray tracing and obtain sound speed at subionospheric height in MSIS-E-90. The result show that the velocity of the disturbances is not in the velocity range of sound speed. Through using the MSIS-E-90 Atmosphere Model and Horizontal Wind Model(HWM), we can obtain the vertical wave number and check the gravity waves can propagate at subionospheric height, to confirm that ionospheric disturbances caused by the explosive eruption is gravity-wave type. Finally, through using Atmosphere Ionosphere Perturbation Model, the result consists with the observation. This work demonstrates that Global Positioning System satellites are useful for near real-time ionospheric disturbances monitoring, and help to understand the mechanism of gravity waves caused by volcano eruption in the future.

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