| Summary: | 碩士 === 國立臺灣大學 === 電信工程學研究所 === 97 === A tsunami propagating on an open sea can induce an internal gravity wave (IGW) into the
atmosphere. The IGW will be amplified due to the decrease of mass density with height,
and will cause a transient vibration in the ionosphere starting at about 150 km above the
sea level. Global positioning system (GPS) and ground-based HF Doppler radar have been
reported to detect such ionospheric variation.
In this work, we propose a complete model which incorporates the loss mechanisms due
to thermal conduction, viscosity, and ion drag. A dispersion relation is derived which reveals
that only a specific range of spatial spectrum can propagate through the atmosphere to
the ionosphere, and IGW amplitude will reach a maximum at certain altitude. Through
ion-neutral collision, chemical loss, and photoionization, electron irregularity is induced in
the passage of the gravity wave. The waveform, amplification factor, propagation speed and
travel time can be applied up to more than 800 km high.
The ionospheric electron density irregularity induced by the Sumatra tunami on December
26, 2004 was detected by satellite-born altimeter and global positioning system (GPS)
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around the Indian Ocean. The model is applied to the 2004 Sumatra tsunami profile which is
restored from the satellite recorded data. The tsunami wave triggers an intenal gravity wave
in the atmosphere, which propagates upward with an average velocity of about 700 - 800
m/s into the ionosphere and significantly disturbs the electron density by 3 to 4 TECU. The
IGW is then trapped at about 400 km height, and moves horizontally with the same speed as
that of the tsunami. Approximately 11 minutes after the tsunami triggers the atmospheric
disturbances, ionospheric irregularity starts to be detected by the satellites that pass over,
and peak perturbation of TEC will be observed in about an hour. The simulation results
well explain the TEC observation in magnitude, waveform, and time delay.
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